Ronald,
As Jim said, I don't think MC could write a tag that differs from what you have in the library. I wonder where MC would get a different genre value if it doesn't come from your file. The only possible thing would be the "Lookup track info from YADB" tool, but you would need the start the tool manually. It does not work in the background.
If you can reliably reproduce the problem with a certain file it would be good to examine the file. For example, a file may have differing tag values in more than one tag format.
MC has a few features that may trigger writing (as it is supposed to always keep the file tags up-to-date). For instance, the manual or automatic cover art or file analyzer tools write tags, but these tools cannot change an existing genre field value to something else.
As also said, you can disable all tag write operations in the general options. When tag writing is disabled MC maintains all library data only in its database (aka library). It may still import external tag changes to the library if you have allowed that in the auto-import options or do it manually by using the "Update library (from tags)" tool.