Command blocks on repeating with 'always active' or 'need redstone' stop working until updated again. This might be occurring when the chunk is unloaded. It's also possible that is is related to other problems with chunks unloading. Had this happen in several worlds by few people.
Linked issues
is duplicated by
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I'm having same issue, see MC-149007.

Need more info on it. Everyone's guess is unloading but cant confirm.

please dont set bug reports to "resolved" just because we dont know how to replicate it. Thank you
When it's resolved as AR it automatically reopens with a response. It's not the end all be all

I understand but it comes across as confusing, I got messaged by a few people saying this bug was resolved already and there fore they didn't contribute to this bug report.

Its resolved as "Awaiting response", its resolved but not fixed
Does MC-86846 describe your issue?

Does this still happen in recent versions? We did fix few problems related to ticking, so this issue might already be fixed.

Still an issue.

In which version?

1.14.4.

I was never really able to reproduce this reliably and now I tried lots of different things in 1.15-pre1, all kinds of combinations between dimension change, view distance change, flying around, reopening the world, loading chunks and quickly unloading them again, laggy and not laggy server and so on. To test this, I had a bunch of command blocks adding and subtracting 1 from a scoreboard. The value stayed at 0.
Does someone know a reliable way to reproduce this? Or is it maybe fixed already in 1.15-pre1?

Was able to reproduce this bug in 1.15.1
What I did:
Generated new world (redstone ready, creative, cheats on, etc.)
Placed down redstone block
Placed command block next to redstone block
Enter GUI and set mode to repeating, unconditional, always active
Entered this command: 'execute if entity @a[nbt={Inventory:[{id:"minecraft:potion",tag:{Potion:"minecraft:mundane"}}]}] run say hi'
Used creative menu to put mundane potion in inventory
No output
Destory and replace redstone block
Continuous stream of output
Not sure what the important part of this setup is, just added every single thing I've down. With more testing this could probably be narrowed down.
EDIT:
Not sure anymore if same bug or different one.
More testing:
Any update to the command block wil fix this (placing block next to it, running a signal in a block next to it, placing an uppowered repeater facing a block next to it)
If the redstone block is placed after the command block is placed this bug does not occur
EDIT:
I believe now this is a different bug. Updating it after it was placed adjacent to a redstone block but before the command is entered also fixes it. Created MC-168681 for this new bug.

Actually Fabian, I thought this was a Bedrock report, as that was what I was searching for. In Bedrock we're still using testfor.

Is this still an issue in 21w20a or later?
What are the steps to reproduce this?