Phone randomly blacklisted???

I bought an iPhone 11 back in April of 2023 straight out from Walmart through Walmart Family Mobile service provider. It is now September 2024 and I’ve had an iPhone 15 for about a month now and decided to sell the iPhone 11. I originally took it to an EcoATM (now that I know what I know it’s all making sense), well when I took it to the ATM I got through the entire process just for it to tell me there was a problem with the phone carrier? I just thought it was due to the phone not being activated. I didn’t think much of it and forgot about it. Fast forward to now, I took it to a phone place in the mall and they told me it was black listed?? I’ve never reported the phone lost or stolen, so this was complete news to me. He told me it was listed as blacklist in September of 2023! However what’s weird to me is, I was and still are able to use this phone! I make calls, texts, it’s now activated (through a new carrier) so how is it blacklisted??! He was explaining how when it’s in that status the phone is basically a brick! Tbh I think at first he honestly thought I was bringing him a stolen device until about an hr later after calling Apple, calling the new carrier and finally talking to the original carrier, who then assisted me on the steps to unlock it… he was finally like well why did they even do that to begin with? YOUR GUESS IS AS GOOD AS MINE SIR! My question is, WHY did this happen? Is this a random thing? Is it a Walmart family mobile thing after you leave their services and switch to another? I mean the fact it’s been blacklisted for an entire year and I had no clue is crazy! I’m just glad I never had true issues with it and took it to the Apple Store! If anyone has answers I’d love to know