I filed FB9045442 about this, and now Feedback Assistant says "Resolution: Potential fix identified - In iOS 15".

Yes, we are seeing the same problem after app updates. Many of our users have to restart their devices to see our extension. These problems have been going on since iOS 14.0 was released. They never happened on iOS 13.x. I have filed FB8889913 and FB9045442 and have been hoping with each 14.x release that it will be fixed, but it still happens on 14.4.1.

I found a way to consistently reproduce this on iOS 14.4.1. If I go to a certain URL (which I would post, but the developer forum software will not allow it), then my sharing action in Safari doesn't show up the first time you open the share sheet, but does show up the second time you open it. This URL has an anchor on the end of it (i.e., it ends with a hash symbol followed by some text). If I got to the same URL with the anchor removed from the end of it, my sharing action shows up the first time I open the share sheet. I have filed FB8889913.