B Boyes

I can confirm that generation seems to now work as expected in 19w03a. A 1.12 save opened in 19w03a, using both the "Optimise World" option and without (done on separate copies), the generation is as expected. Hopefully the OP can confirm also.

Similar to what João has noted, I can add that a 1.12 generated world, updated to 1.13.2 and then opened in 19w02a, exhibits the same behaviour where the 1.12 generated terrain was unexplored in 1.13.2.

It seems that 1.12.2 generated terrain that is re-visited in 1.13.2 remains unchanged in 19w02a, but exploring outside of this causes world regeneration, leaving only blocks placed in what was minecraft:air in 1.12, to be left intact.

Can confirm in 1.13-pre5.

It seems that if you place a bed, then break it, then place a shulker box where the bed was, you can't open the shulker box and it will be deleted when you mine it. (This is in a survival world updated from 1.12.2.)

I noticed a similar situation in single player survival. Blaze shoot fireballs from roughly 50 blocks away on hard difficulty. When further away they remain idle, but attack immediately when they target the player at roughly 50 blocks.

Now confirmed for water touching signs in 0.13.0 on iOS 9.1 too (iPad 3).

Signs now behave like torches if placed on the side of a block for source or flowing water; breaking as soon as the sign is placed. When placed on the top of a block (a free standing sign) the water is held back as expected.

Confirmed for zombies and skeletons for me since 0.12.1 on iOS. Both drowning and falling mobs no longer drop expected items, except for armour. Also, burning undead mobs no longer drop items either when exposed to sunlight.