I’ve had this happen to me twice now in the last week. Once during Evacuation, once during ordinary Hunt skirmish.
When joining a game in-progress, instead of gaining control of the AI Monster you’re stuck in some weird placeholder first-person Monster camera that can’t jump, feed, attack, whatever.
You can however still walk around, be affected by gravity and even trigger birds and Harpoon Traps.
I’m not able to purposefully reproduce the issue, sorry. It just happens occasionally and when it does I try to just make my presence known to the Hunters with spooky bird alerts out of nowhere.