Upon creating entities with certain tags, you can't do anything with them. It's... oddly specific too. For example, I summoned an entity using "/summon minecraft:chest_minecart ~ ~ ~

{Tags:["CakeSliceCovert","Mainhand"]}

". Then, I checked its tags and it returned "Minecart with Chest has 2 tags: CakeSliceConvert, Mainhand". So after this, I finally did "/kill @e[tag=CakeSliceConvert]". It says "No entity was found". However, say I change that to "/kill @e[tag=Mainhand]". Suddenly it works. So here's how you can recreate this... very odd bug.

 

1.) Be on 1.17

2.) Run "/summon minecraft:chest_minecart ~ ~ ~ {Tags:["CakeSliceCovert","Mainhand"]}"

3.) Check its tags by looking at it and doing /tag [its UUID] list to confirm that it does in fact have these two tags

4.) Run "/kill @e[tag=CakeSliceConvert]". Nothing will die

5.) Run /kill @e[tag=Mainhand]. It should now die

This is so oddly specific, thought I'd report it though!