Billed for 2-3 lines when I have only 1 line

In September, with the release of the new iPhone 15, I decided it was in my economic interest to upgrade my cell line plan to Go5G Plus, to take advantage of service promotions and device discounts. The promotions and device discounts have been as promised, and I haven't had issue there.

My issue is that for 2 billing cycles now (and I suspect a 3rd coming soon), I have been charged as if I have a family plan of either 2 or 3 lines. I only have 1 line and have always only had 1 line.

Now when I talked to support via telephone, I realized it's important to whom you speak.  I had credits on my account from the trade-in I did back in September. So when I was overcharged for 2-3 lines, some reps would explain to me that everything is actually correct because (bear with the leap in logic) I technically didn't owe because the credits covered even the overcharging.

I finally got a rep the first time who realized right away what was going on, and I was properly given a credit adjustment.

To my astonishment, it happened the second billing cycle. The first rep I spoke to gave me the spiel of how because I technically did not need to send in a payment to T-Mo due to my account credits covering the overcharge, that my bill was correct. I had to escalate to a supervisor and she realized the problem after I carefully explained to her the situation.

I’m afraid this is going to happen a third time.

Has anyone experienced something like this? Part of me wonders if this is an intentional scam to drain your account credit from your device trade-in.