Thanks to the work of @kimberlietx & @megachile many of these galls have been found and put on iNaturalist and they have been confirmed to be caused by seemingly undescribed gall mites. Here is a great example of what this species looks like when cut open with lots of detail and commentary:

https://www.inaturalist.org/observations/61068804

And here is a webpage dedicated to the species on GallFormers.org :

https://www.gallformers.org/gall/1182