Hello Tim,
I'm still getting a scare every time from the red area and popup when I'm flying through a NOTAM activated area that's not active at the moment. The red is a surprise since you didn't see any red while planning, but grey. There are many of these regions in the Netherlands and even more in Belgium. I really hope this will be changed soon.
If you are afraid of clearing these areas, I think you could show it as grey and present the user with extra information on when the NOTAMs were read (e.g. NOTAM refreshed 1h 21m ago). Maybe make it red anyway if that was more than 4 (or configurable) hours ago.
To prevent this being a surprise while flying, when the user goes to navigation mode, you can give a popup if an area is now active while it was not active while planning, or if the NOTAM's haven't been read recently.