From the above description one thin is not fully clear to me, so aksing specifically: Is this a regression introduced with the 4.19.194-1 upload or can you reproduce the issue earlier?
I had no spontaneous reboots within the last years so yes, the issue seems to got introduced with one of the recent kernels in the last few months.
If a new regression introduced, can you possibly bisect to the introducing commit? Can you reproduce the issue with recent kernel from unstable or buster-backports?
I'm going to try a more recent kernel from backports or unstable to see if issue still persist. As I this issue only pops up while using bonding interfaces on a multiport GBE NIC I wonder if there is "just" a configuration issue with the bonding interfaces. Cheers spi