@MartinDEE, I'm having the same issue. Diversion and Skynet installed to an RT-AC86U running Asuswrt-Merlin's latest firmware. Both scripts are up-to-date.
At first, achievements stopped unlocking on my Xbox One X. I then gathered all the domains I could find that were required for Xbox functionality and added them to a hosted whitelist in Diversion (processing it, so Skynet would be able to "see" it as well). The full list of domains I uploaded can be found
here (compiled after going through a bunch of threads on Pi-hole's subreddit and also by combing its official FAQ - same link shared by
@Vexira).
Achievements started unlocking again, but only new ones - the ones that should have already been unlocked, remained locked. The "local cache is full" message also kept popping up.
You mentioned solving the issue by "setting bing.com to strict.bing.com in dnsmasq config". How do I do that in Diversion? Do I just add that domain to my hosted whitelist? Neither "bing.com" or "strict.bing.com" are there.