My unique maps disappear rarely

I had placed markers on 6 unique maps:
- 1 Fractured Lake
- 2 Viridian Wildwoods
- 2 Abyss
- 1 Untainted Paradise
I fully remember their location and marked them with names yet they disappeared after a day of not playing. I noticed this so I figured if I restarted the game they will return but they didn't. So I just waited a day and they still haven't returned. This isn't an inconvenience to me but I just found it a weird bug. They revert to being normal maps with nothing on them but they keep the tag I've placed on them. It's happened in two different days.