I think it MOSTLY obeys that I have turned off "Degrade tiles" but I still find a stray degraded tile here or there. My suspicion is that there's a code path where the setting is either not checked, or maybe there's a time when the value has not been checked/loaded yet that tiles degrade, before it checks/loads and then stops doing it.

I looked at the code and it appears it might be that "degrade tiles" is not observed when the tile is reinforced concrete. I assumed unselecting "degrade tiles" would stop ALL tile degradation. It appears you need to uncheck "degrade tiles" AND check "do not degrade reinforced" to keep all tiles from degrading. I would recommend having "degrade tiles" be the master switch, and only if you allow degrading tiles could one then turn off reinforced checks.

Thanks for this post. I was having the exact same issue and it turns out it's due to the exact same reason: I assumed that if "Degrade tiles" was unchecked, the state of "Do not degrade reinforced tiles" would be irrelevant, I thought the "Degrade tiles" option was a master switch like you did. That should definitely be changed to be the case.