Purchase unlocked iPhone that says "SIM locked"

It's working on Mint Mobile as far as I can tell. Is there any potential for issues in the future? Is there any actions that should be taken right now?

Where does it say that, exactly?

Settings -> General -> About -> Carrier Lock: SIM locked
Clicking into it it says "This device might be restricted to the original carrier and/or have certain capabilities limited.
Contact your provider for more info"
The seller told me it's because the phone was unused for 90 days so there is a temporary lock. After 90 days of usage, the temporary lock should go away.

The seller lied to you. Phones are never "temporarily locked" to the carrier. The phone must originally be from T-Mobile, because Mint Mobile uses T-Mobile towers. So the only SIM locked iPhones which can be used on Mint are those that are originally from T-Mobile. As long as you stay with Mint, there shouldn't be any problems - unless the phone gets blacklisted at some point in the future
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