@iddqd & @ubermutant : I doubt that this is a borked flash. I own 4 TOs and all behave in exactly the same broken manner. The only slightly non-standard thing I do on my boxes is install fix-switch.
During my initial wizard-based install I define a LAN distinct from the default 192.168.x.y and indicate my locale as USA (not the Czech Republic). I suspect that LuCI fails to seed consistently all of its fields from authoritative sources. Rather I would guess it has some fields seeded with values that match the wizard’s defaults. Were that the case then when I make a change via LuCI it would write back stale values leading consistently to a broken configuration.
Probably not related to your problem, but the issue that fix-switch resolves was fixed in Turris 3.5.2 release. If you’re up to date you shouldn’t need fix-switch anymore.