Yes! Exactly the same problem with posting images from Photos app to Facebook - only. Can select images to post but their is no longer any 'done' button to enable completion of the task. This is ever since my iPad Pro updated overnight to iPadOS 16.3.1. I know because I post regularly. On the evening of 12 March 2023 I posted to FB and on the morning of 13 March I could not. My OS updated automatically overnight.
The same device can still post to messenger and to IG. My other Apple devices on earlier OS (iPad Air 2 & iPhone 12 Pro Max) still work fine to post to Facebook.
Have tried every fix recommended by Apple and Facebook. I believe this is a system-related bug.
I would love to know how to revert to an older OS on the iPad Pro while the issue is being addressed.