revert to 15.1 from 15.2

15.2 has broken some of my software, I'd like to revert to 15.1. I have a TM backup from before I installed 15.2, but I can't restore it. I booted into system recovery selected the backup date but it won't restore it - it wants to install a new version of macOS and use migration assistant. This is not helpful. Does anyone know why it won't restore the previous version? Possible workaround? TIA.