Sea grass changes to air pockets when upgrading from 18w16a to 18w20a+

It affectes the 18w20c snapshot too.

Like Frank said, replacing them with water is the only reasonable thing to do in future world upgrades. As it stands right now, this breaks a lot of worlds' oceans etc.

This bug doesn't break anything since Kelp was introduced in the 1.13 snapshots.

For all the players on here, it's not our place to impose our remediation beliefs on Mojang. We find bugs, they fix them. You can analyze causes of them, but postulating about how Mojang SHOULD fix something is for Reddit, not JIRA.

Additionally, Kumasasa is right. All 1.12 worlds will be fine upon updating to these snapshots - we accepted the risk that any potential worlds would be broken along the way when we started playing on snapshots. Comments about how Mojang must fix this are misplaced on these threads.

When this is fixed, will the air pockets still be there? I'm gonna go mad if so because I'll have to do a lot of watering.

Confirming the issue affects 18w20c, although seagrass in newly-loaded chunks is fine and generates normally. 

Also, to anyone wanting a temporary fix to this issue, you can use the fill command to replace the air with seagrass or water (though if you replace with seagrass it will put two 1-block high seagrasses on top of each other). 

There's a block limit to the bounds you can put on the fill command, but it should be something like this:

/fill 0 50 0 50 40 50 seagrass replace air  OR  /fill 0 50 0 50 40 50 water replace air

You'll have to do this over and over again, but it's better than nothing (especially since hostile mobs spawn in the air pockets). 

Our major interest is getting the upgrade from 1.12 -> 1.13 working correctly.

Please be aware that due to the nature of snapshots and the potential cost/time investment to upgrade between snapshots themselves we sometimes will not provide an upgrade path from one snapshot to another.

If you really want to have certain things looked at, the best way to help is making sure we have a clear set of reproduction steps, like:

1. make world in <public available snapshot version> with seed: thisismyseed

2. /tp x y z rot

3. load the world in <public available other snapshot version>

4. See <what is broken at the same location>

Here are steps to reproduce:

1. make world in snapshot 18w16c with seed -1259081801.

2. explore an area of ocean with seagrass present (this seed will spawn you on the shore directly next to the requisite ocean).

3. load the world in 18w20a, 18w20b, 18w20c, or 18w21a.

4. travel the area explored in 18w16a.

5. see seagrass in this area has been replaced with air blocks. 

6. travel further in the ocean to unexplored areas.

7. See seagrass is present and normal in newly explored areas. 

Affects also 18w21b.

I think it' s important  : the items of seagrass in a chess disappeared too...

Still in 18w22a

This bug was NOT fixed in 18w22c. Sea grass and salmon still not converted correctly.
Console output:

Invalid statistic in C:\YAMS\servers\7\world\stats\c20e3a0f-e2c4-468d-a123-6a0b3af0c6e6.json: Don't know what minecraft:salmon_mob is
Invalid statistic in C:\YAMS\servers\7\world\stats\c20e3a0f-e2c4-468d-a123-6a0b3af0c6e6.json: Don't know what minecraft:sea_grass is

Use Rat DOg's steps to reproduce.

Yes I confirm the bug is NOT fixed in 18w22c : seagrass and item of seagrass always disappeared...

I have a world saved in 18w16a and when i opened it in 22c all the air blocks were still present. So I believe that there is still a problem.

Well they did say that they could not promise that snapshots could not be updated so on that my test may be flawed.

For everyone repeatedly commenting, please remember that comments are meant to be used for constructive additions which aid in developers understanding and resolving the issue. This is not a place to vent frustration or repeat evidence which has already been gathered or stated.

The devs are aware that this issue persists, they have stated that it is a very low priority as it does not impact stable versions of the game, and will likely not invest time to fix the issue. We took the risk when we started a world within a snapshot version.

This bug can be solved by modifying the NBT tree of the world:

1. Back up your 18w16 world.

2. Change the value of each (string) tag containing minecraft:sea_grass to minecraft:seagrass.

3. Change the value of each (string) tag containing minecraft:tall_sea_grass to minecraft:tall_seagrass.

4. Open Minecraft and convert your world to the last snapshot.

It's feasible, although rather slow, with NBTExplorer.

Hope this helps!

Can you explain where do you find the (strings) tags ?

@DieuRautavaara

It seems the last pre-released has solve the problem, but for future reference, the tags are:

• In the various elements in Level.Sections.[].Palette in each chunk of each region for the sea grass in the world, with the name Name.

• In the various elements in Level.TileEntities.[].Items in each chunk of each region for the sea grass in chests, with the name id.

• In the various elements in Inventory in the player data for the sea grass in the inventory, with the name id.