Go to AdGuard's main screen and tap (Update Filters) in the right upper corner. Check if your problem is solved.
Go to Settings → Safari protection → User rules and check if there are any rules that contain the problematic website's name. Disable them and check if it solves the problem.
Go to Settings → Safari protection → Filters and disable all filters to see if that will fix the problem. If it does, start re-enabling them one by one, checking after each filter if the website(s) are displayed as they should. This will help pinpoint the problematic filter.
If it's one of AdGuard filters, let us know about it by contacting firstname.lastname@example.org
, and we will fix the problem.