Instead of telling the customer that thete is a problem, they chose to do nothing. I am very dissapointed and am seriously thinking of switching carriers.
There is indeed a known limitation. If you pay off a device, it creates a credit balance on the account if you have a bill out. If you still pay the bill as printed/posted on mytmo, there isn't an issue. If you don't end up making the bill payment in addition to the device payoff, things get a little wonky.
It's definitely frustrating and we are aware of the posting issue, the good news is you aren't charged anything additional and nothing extra has been applied, it just posts inconveniently.