Gregor, the map insert on the area-specific pages of the Protected Planet website https://www.protectedplanet.net/en (with the WDPA stuff) appears to crash every now and again, resulting in the behavior you explained.
I have been in contact with UNEP-WCMC who manages the Protected Planet website many times and informed them about the bug. The situation always gets fixed, but it also tends to reoccur, so they appear to have an unresolved bug that resurfaces from time to time.