In your second paragraph, did I miss any other problems with uploading files in 4.8.1? For example, are there any issues with getting the files to upload as well? Or are the issues only with tagging multiple files that you upload in a dataset?
Demo Dataverse is running 4.8.1, so I used it to reproduce some problems I think you're describing with tagging multiple files at once. The tag box doesn't stall for me, but there are times when I'm not sure if my tags were actually applied. Often trying to apply tags to the files in bulk a second time works, or, like you said, tagging them individually always works.
I couldn't find a github issue describing any problems related to tagging 2+ files (although I know it's something the design team here at IQSS has discussed). Could you open an issue and describe your workflow? Or maybe we could arrange a video chat so we make sure we understand.
Please let me know if I'm missing anything.