@dc42 Turns out I had accidentally gotten the others "working". My scripts attempted to do the purge in Tpre, when extrusion is disabled. The earlier filaments didn't form a plug sitting in the hotend, just oozed all over instead.
Fillamentum filament has always given me issues with forming plugs, even in the Prusa MMU1 and MMU2. So in the broken macros, it was cooking and formed a plug/clog.
I think I either need to attempt to get the purge and 50mm retract out of the hotend completely working (without grinding during the purge and forming decent tips that will re-feed later without grinding), or switch to just retracting ~5mm - which is the max for the chimera heatbreak before plugs form at the top of it. That method might be more difficult to control oozing.