Hey there, I had a similar problem like you, but instead with my super commando and arctic trooper and it's a bit different. Here it is:
https://account.xbox.com/en-US/screenshot/f0a5e7ed-2cfb-4b3c-a1b0-b02acfe84212?gamertag=charlestallwolf&scid=00000000-0000-0000-0000-000000000000
https://account.xbox.com/en-US/screenshot/4056e8bf-759c-4183-b2ac-7e85ef4a80a5?gamertag=charlestallwolf&scid=00000000-0000-0000-0000-000000000000
https://account.xbox.com/en-US/screenshot/77f21582-ef3d-443c-a479-e1c6bff2554f?gamertag=charlestallwolf&scid=00000000-0000-0000-0000-000000000000
I promoted my Arctic Trooper and Super Commando from elite to super elite, but every time I went into multiplayer, I would keep the coins gotten from the promotions, but my characters would be elite again (not to mention my Arctic Trooper was level 11 elite). So I saved my game after I promoted my characters for the third or fourth time to see what happens, and when I went into multiplayer, I said they were master rank. So I played with them a bit, and then they reached levels from -7 to 1 (https://account.xbox.com/en-US/gameclip/1a3fd3ca-c6d5-486a-8b11-9b9190f00fe8?gamertag=charlestallwolf&scid=00000000-0000-0000-0000-000000000000). So when I both got them to "level 1 master," I returned to my backyard, only to see them with no rank plate and they were "level 46," even though they were master rank. I know this is very long and not believable, but trust me, this is real. BTW, I'm on Xbox One too, I you couldn't tell.