Never had this problem after using constantly for 1+ year until today:
Click "Send"
Choose "Contact"
"Select From Contacts"
At this point, I have always been able to type in a Mixin ID from the messenger app and the name of that user would pop up. Now, nothing comes up. I tried multiple correct Mixin IDs on multiple different browsers too.
Never had this problem after using constantly for 1+ year until today: