It looks like the same kind of issue as T57463, where LiquidThreads loads the core 'mediawiki.action.edit' module on its pages. The module was changed a bit recently to match the update of edit form to OOUI (T111088) and presumably those changes were not reflected in LiquidThreads. It's LiquidThreads responsibility to keep its markup compatible with the core module. Or just stop using it – to be honest I don't see what it's even loaded for, as the module doesn't do very much.