Normally, when you run the /xp command in the console, you can only use negative values if you are taking levels. If you are not taking levels, it will give you an error message saying that it could not give a player a negative amount of experience. That part works fine.

However, if you do it in a command block with the tag entity selector, the error message will tell you that it could not find an entity that fit the selector instead. This can be reproduced by running /scoreboard players tag @a add tagged in the console then /xp -1 @a[tag=tagged] in a command block. It fails as expected, but tells you that it couldn't find @a rather than the actual error.

EDIT: I made a mistake; the bug only occurs with the tag entity selector.

EDIT: This actually applies to all commands and errors, not just the one I mentioned.