here’s an interesting tool that allows for comparison of different sources of administrative boundaries: https://www.geoboundaries.org/visualize.html?country=BEL&mainSource=OSM-Boundaries&comparisonSource=GADM+v4.0.4&mainLevel=1&comparisonLevel=1
it looks like every set is going to have their pros and cons, and GADM seems to actually be better than most in many cases. for Belgium, this tool says that the resolution of the latest GADM set is 269.7 m, versus 33.3 m for OSM, but resolution varies by country.
(so the simplification of the boundaries that i talked about seems to be due to the resolution of the original GADM set itself and less likely due to anything happening during ingestion by iNat.)