Looks like it affects pages enabled (without wgFlowOccupyPages) before https://gerrit.wikimedia.org/r/#/c/197640/ .
a5bc9f49cd01e49cb78924686c7f35b03f45d8ea triggered this (although making save and read for Revision consistent).
Another reason for https://www.mediawiki.org/wiki/Requests_for_comment/Content_model_storage and always saving the content model regardless of whether it matches the default.
All of these are fixed now. I reran my detection script and found no broken pages.
This appears to have been an old issue with how Flow boards were created. It was fixed in https://gerrit.wikimedia.org/r/#/c/197640/ , but that only fixed Flow boards created after that patch. Flow boards created before 1.25wmf23 (which was deployed between March 25th and April 1st) had bad content model fields in the database. However, these pages kept working essentially by accident, and then broke recently because https://gerrit.wikimedia.org/r/#/c/222043/ (merged in 1.26wmf13, deployed between July 7th and 9th) removed the accidental behavior these pages were relying on.