My guess is they'll eventually catch up with it.
If you don't return all the devices, the account will definitely get flagged. They'll probably look into things and say "oh, this IMEI is what was missed."
There's probably no loophole to find. In fact, it may cause more headaches to get it unblocked if they catch up with you before you have a chance to pay off the device. Accounts sent to collections can be quite a headache to deal with when straightening out since the collection agency will say talk to T-Mo and T-Mo will say talk to the collection agency (the collection agency truly are the folks to talk to -- but, they don't care about anything besides getting money)
smply is correct, it probably just hasn't caught up. It's not an automatic thing for the device to be blocked as soon as the account goes to collections, we have seen it take awhile after. It's especially rough when we see people who have bought the device second hand, used it for awhile, and then the system blocks the IMEI and that person is left frustrated because they have been using it the past few months no issue only for it to be a brick one day.
So if you can, I'd get everything in good standing!