Well, after poking a bit, I realized I had modified this library about a year ago to support "LCD Any Pin". I put the original back and that solved the problem. I'll have to do some digging to figure out what mistake I made, but it's clearly my issue so thanks for thinking about it and I'll take it from here. I have not noticed it in all this time, likely because I've been using 18 series parts, which don't use this lib.

Thanks also to Darrel for the gentle hint around the place to start looking.