Why does my iPhone SE restart after a day of use with a kernel panic?

If it is in fact like the one you posted, could either be an easy fix, or real bad news.

The actual log refers to a SEP Panic. The SEP is the Secure Enclave Processor. It's a bit of the CPU where the encryption keys for unique phone data are verified. Things like Touch ID pairing and encryption for your phone are linked to the SEP. It also runs its own operating system called SepOS. Which is likely where the panic stemmed from. A SEP Panic would mean either there is damage to the phone related to these communication systems. Data the SEP needs to access are not usually stored in the CPU, but a tiny chip elsewhere on the board. So lines of communication between the CPU and the location of the data are interrupted. Or the firmware/iOS are corrupt.

Based on the fact that this occurred after a downgrade to the operating system, I am likely to believe this is firmware/OS related. Or at least it would be a good place to start troubleshooting. I would recommend backing up any data and doing a restore through iTunes. Or beyond that via DFU mode to rewrite the actual firmware.

I would still want to be sure this is the same panic you're having, since I rarely pay attention to the bug codes, so be sure to verify the panic string in your panic is the same SEP panic as is listed in the log you linked.