Whenever you quote a message, if you delete a line to quote only part of it, for example, or write some text before the quote, or even just ctral+a the whole message, the @-mention goes to the end of the message and deletes the last characters of the message as well. Trying to edit or manually move the @-mention doesn't work at all. Sometimes, if you edit the message after sending it, even trying to remove the @-mention entirely makes it show up again.
To sum it all up, quoting with @-mention doesn't work properly.
Hey Gabriel! Our Product Team is aware of this issue and currently working on it. If you have some examples you could share, please open a case and include JIRA GLIP-17881 in your case notes along with screen shots :)