Same issue here. After updating to release 3.7 my content pipeline extensions don’t work (edit: the content still builds but mgcb crashes or shows missing/invalid extensions). I have upgraded everything to 3.7 and cleaned/rebuilt. Upon a great deal of experimenting, it seems that extensions that rely on types in another assembly are the ones causing the problem.
Update: I poked at the Pipeline source and created the following PR https://github.com/MonoGame/MonoGame/pull/6566 that fixes my issues.