Well I for some reasons couldn't recreate the bug - but from the screenshot one of my friend gave me it couldn't have been due to a large fall due to the location where he sank. Tried to fall from a high place and didn't sink either. My friend said he was just walking around lol
Oh well don't think I'm going to be updating this game anymore, but I guess I will do as you suggested and change player's rb2D to Continuous and reupload it.