I’m guessing you have your SD card set up as internal storage as this is a known issue with cards formatted as internal storage - is that right?
If you do have the card set up in that way, it’s recommended to instead set it up as removable storage. It’s less convenient in some ways, but it’s not susceptible to these kinds of corruption and system errors.
Side note: have you tried using OsmAnd for navigation? While it’s not as full-featured as G00gle Maps (especially if you rely on traffic data), it saves maps locally and runs a lot leaner, in my experience. It may alleviate the reboot problem.