hi Schabulsky, we make different method to add tag on latest version.
its because we notice people may have thousand of tag, and if we load it all, it will be quite catastrophic and cause the issue.
to add tag on themes right now,
1. you will need to enter the name of tag and wait until dropdown showing. and after that select 1 tag you want to add.
2. or other alternative way you can try to directly write ID of those tag. and right before you leaving the tag, please press ( comma )