iOS DNS VPN not resolving mail server

This issue has been coming and going for more than a year (and it has been a while since last time): Certain names suddenly don’t resolve via cloudflare-dns ( [iPAD, iOS]. In our case mail.‘domain’.com suddenly doesn’t work. Turning off cloudflare vpn fixes the problem. I know that and can do that but for the users its a real pain in the rear. Usually it takes a couple of days, then it’s working again. Does not seem to have anything to do with the iPad (power cycle makes no difference, all updates installed, etc.). For now we just ditch cloadflare-dns, but we’d like to get it back.