Update on the issue,
Since the night of Monday 7th December, the error message “You can’t use that item” is not showing anymore, instead the error message “Your level is too low to use that item” is showing.
But it’s still the same issue following the same mechanic as detailed in the original post, the text of the message changed but the issue is the same.
Even items with “Use:” effect having a low level requirement (example with the garrison building plans) display that error message with a character that is over the displayed level required in the tooltip of the item.
It still affects pets that afaik never had any level requirements in order to be learned …
This change might show that the Devs are trying to fix it, I just hope they’re not “finished” with it because it still affects a lot of players …
Edited OP to reflect changes in error message