iPhone SE (2020) I brought brand new direct from Apple and paid off, IMEI blocked

As the title says, I have an unlocked iPhone SE (2020) that I bought brand new directly from Apple.com and it has been paid off in full. I use it as a backup phone for my personal line (T-Mobile), as well as occasionally for work (Verizon).

Last week I inserted my work Verizon SIM card from my work supplied Android into my unlocked iPhone SE and was unable to make calls or connect to data. When I put that SIM back into my work-provided Android, the SIM didn’t even activate the service; I called Verizon and they basically treated my business phone number as non-existent/disconnected. I eventually had to go through my work’s IT, who told me the line was disconnected due to “non-use”, but it made me a little suspicious.

I checked my iPhone’s IMEI through Swappa and it comes back as “Blocked”; similarly, when checking on T-Mobile’s website, I get the following message:

This device is currently blocked from use on our network. Contact the original wireless provider for information on the status of the device.

(This phone was originally activated on T-Mobile)

Just for the record, I checked Verizon’s website; I get a similar but shorter error: Unfortunately, your device cannot be activated on our network.

So my questions are:

1. Why is my phone’s IMEI blocked while the phone is fully paid off? My suspicion is that putting in the most recent Verizon SIM card from my work triggered some kind of fraud prevention as the line associated with the SIM card also immediately disconnected.

2. How do I get it unlocked?

3. How do I prevent this from happening in the future? Can a phone’s IMEI really be blocked by inserting a SIM card?