When I tried to make a function, I ran into an issue where if you use multiple tags in a selector, it will only care about the last one, and disregard the others.
Example:
I want to execute the command '/say hi' on each armor stand with the tag 'test1' AND the tag 'test2'.
I use the command:
execute @e[type=armor_stand,tag=test1,tag=test2] ~ ~ ~ /say hi
However, not only the armor stands with both tags say hi, but also the ones with just the tag 'test2'. This is a big problem for people like me who are depending on being able to use multiple tags.
I also tested this in the chat, command blocks and advancements, but they all got the same result.

P.S.: I know this bug has been reported earlier, but that was back in 2015, and has been resolved, but reintroduced since then, and not been reported yet.