Hello Yhaka,
Thank you for reaching out to Apple Support Communities.
Let's attempt to clear your website data to improve performance.
"Clear website data
You can clear website data occasionally to improve Safari performance. If you still need help, follow these steps:
- Go to Settings > Safari.
- Tap Clear History and Website data.
- Tap Clear History and Data to confirm."
More information can be found here: If Safari displays a blank page or quits on your iPhone, iPad, or iPod touch
If the issue persists, can you provide a few more details? When did you first notice this? Which website are you attempting to view a video on? Does this happen on multiple websites? Also, what exactly happens after you tap on full screen?
Good Luck!