To reproduce, create a mcfunction which summons a tagged entity and then looks for it. When executed directly it works as intended.

When executed as a reward from an advancement, the pig can't be found within the same tick for some reason.

• • • This is a critical bug, as it breaks all datapacks that use advancement triggers for mcfunctions to run on summoned entities in the same tick. ***

summon minecraft:pig ~ ~ ~ {Tags:["panic"]}

execute as @e[type=pig] run say pig

execute as @e[tag=panic] run say panic

tag @e[type=pig] remove panic

There will be no say output this same tick.

This used to work within the same tick for 1.15.2 and earlier.

[Mod]tryashtar's comment below "Can't reproduce" was before my edit above to include that the mcfunction fails as described when triggered as an advancement reward. I'm not aware of tryashtar attempting to reproduce since I added this detail.