Out of the box, this LTE6 had two LTE interfaces on it, lte1 and lte2. After upgrading the LTE firmware on it, one of them disappeared. Is this normal? Or did I brick something?

Chateau LTE6 has one modem only. Why did you have two interfaces at all? USB modem attached?

I’m not sure. No second modem attached. I’m wondering if the “second one” showed up during the upgrade and it was really just a “duplicate” of the first one?

I've seen the happen in past, although not recently.

My guess is during the firmware upgrade, the USB device-id change for a short period...RouterOS thinks that's a new modem. After a reboot, there should be only one LTE device & if so, it should be fine. Assume it work and there is one again, if it's named "lte2", you can just rename it "lte1" again if you want.

Yup. It was named lte2 - but I did a “reset” on the interface and it came back as lte1. Good to know I didn’t break anything!