There could be a few reasons why this is happening:
- Please try disabling Auto Lock (which can be found in Settings > Display & Brightness > Auto-Lock) and see if this fixes the issue.
- Also it’s worth checking your Safari settings – In Settings – POP UP BLOCKER should be OFF
- Can you check whether you have installed any kind of ad blockers?
- If you are using an iPad/pro, use only in “full screen” mode and see if that makes a difference.