In the meantime you can change the User Agent to another browser and it will work. The issue can also be solved by Facebook allowing version 16.5.2(a) or Apple reverting the User Agent to the previous one. The User Agent setting does not affect security and is used by websites to identify the browser used. To temporarily change your User Agent in Safari:
- Enable the Develop Menu by going to Safari > Settings > Advanced > Show Develop Menu in menu bar.
- Then go to Safari > Develop > User Agent > Safari - iPad OS 16.4 iPad
Once the issue is resolved by Facebook/Apple, I recommend changing this setting back to Automatic.