I have the exact same issue with my pixel 4 so not just a Samsung issue. Thought it was maybe latest update on my pixel that caused it but clearly not. Anyway, not a massive issue considering everything else going on in the world.
It has more to do with Facebook messenger itself because of a recent update Facebook released. There are two things you can do to fix this situation. The first is if it's a website you're sharing; just copy the URL of the site you want to share and paste it in messenger. That still works.
Also if you want to share a picture or a video from your camera roll, there's a button at the bottom of messenger that let's you access your camera roll to share the pictures and videos you would like to share. That still works also.