For me when I was trying to rescue my dinos from redwood and i went with my Parasaur and i got attacked by these guys so i went home and the leech was on my screen instead of on the Parasaur so i called my cousin but he was no help(sorry if you read this lucas)and it turned out when i removed the leech from the Parasaur it was gone from my screen so I think when there is a leech on your screen but theres one on your dino then I think you just remove the leech from your dino and it will probably be gone from your screen hope this helps
These are not as rare as you think. Every other I get yoinked off a mount by a kapro I end up running into at least one. That being said I don't get yoinked off mounts often but when I do in the swamp I find a leech. After puttering around on my pelagornis I managed to find enough to get 30ish leech blood.
To the post that said they weren’t sure if swamp fever exists in mobile, it does. I’ve gotten it on mobile. Also I was killing animals in the swamp with my dire wolf and it said killed infected leech in the kill feed.