Steps to reproduce:

1. navigate to host screen
2. select on or several hosts
3. Click on mass update
4. on tab tags, select replace
5. specify you tag/value
6. apply

Result:
Tag is correctly add/updated but all others tags present on hosts are removed
Expected:

Only selected tag should be updated, without deleting other tags as per documentation:

Replace - remove the specified tags and add tags with new values