I have had this problem on multiple routers now, the most recent happening twice a day.
Hotspot login screen stops working, login screen hosted on remote server and walled garden setup for it. Local login page on mikrotik redirects to remote.
When this happens a deletion of the masqurade statement and re-creation of it fixes the issue. A factory reset and reconfig fixes everytime as well.
I have also had the same issue with not hotspot setups, same fix applies.