When replying to a topic/post and typing @ a popup no longer shows with a list of participants to choose from. One now has to type at least the first letter which pops up a list of "all users" starting with that letter.
What changed?
Win 11 Home, Firefox 132.0. and 132.0.1.
Good. Hopefully you will soon also be able to type a colon character ":" without having to fight with a stream of "emoji" suggestions.
I don't agree 
If I use an colon in a sentence, it will always be followed by a space and the emojis disappear immediately.
You can also complain about the # which pops up a list of categories.
@sterretje same for me (Chrome/Android).
I think you are correct, the list used to appear as soon as you typed the @ and showed other contributors to the topic.
I do have a problem with emojis. For example if I try to select the wink emoji 
or the thinking emoji 
or the laughing emoji 
it sometimes gets it wrong.
I'll try the wink emoji again
Oh yes, that is also irritating. I use the "#" often when I refer to a post by number. I am no great fan of these keyboard "helper" functions. It gets worse. Trying to type code on an Android tablet (say Samsung Notes app) is a misery because of you are continually fighting against unwanted capitalisation etc.
Thanks @sterretje. It is working again after the bug fix was pulled in by today's update of the forum's Discourse forum framework:
Maybe you have the habit of adding a space before the colon?
If you use the colon in compliance with English language rules, without a leading space, the emoji markup picker will not be triggered.
I also sometimes fight with it in this case. However (unlike the emoji picker) I do use the category markup picker feature regularly and find it quite useful. I think the markup provides a nice way to referring to forum categories in our posts, but I would have quite some difficulty writing the correct markup manually.