User tag doesn't work (mostly in embed messages)

When some users on my server try to tag anyone, instead of a name coming up and that user getting a notification, the tag shows up as "<@108554546283806720>" or some other long line of numbers, depending on who they try to tag and I get a notification (I'm the server owner). They've told me that they don't have this problem on other servers. This is a problem since 2018, we need a fix.