The "new agents" was a rewrite of the code that drives the testing and records the page results. Instead of running on Windows-only and intrusively intercepting the OS API calls it is now cross-platform and uses the supported dev tools interfaces, netlogs and traces. As part of the move we also switched to running on Linux VM's instead of Windows which reduced the overhead and made it possible to get accurate performance stats out as well.
What kind of "drop" are you seeing with Brotli? There have been a few issues with the response headers reporting differently, specifically cases where the same header is repeated (ended up with a newline in the data) but we'd like to fix any cases that can be identified.
I still don't understand the reported drop in HTML, JS and CSS bytes (while the top-level bytes remained consistent) which looks like it may have also hit images on mobile. Best guess right now is that the content type matching somewhere is missing some resources but I haven't found where yet.