I actually did see this happen recently - I am playing on Firefox on Windows. I think it was a pretty rare case though. I believe it was two birds that knocked into one another, but specifically what happened was that they had *both* been newly created and were both in that initial ascent, which I assume is coded not to merge for a short time to allow the bounce thing? But it was bizarre since they did bump into each other and land apart!
Curious, curious. I actually coded it so animals *can* merge in midair. There's really no cooldown timer on any of that. However, usually that bug is caused by the browser not having enough memory for godot to process all of the physics interactions (the physics engine is pretty greedy and not well optimized apparently). Some of my friends found that it can occur occasionally if they have too many other tabs open in their web browser, though I think it was more often reported as an issue with Chrome rather than Firefox.