Was not sure the correct way to escalate this quickly enough. I unfortunately discovered this issue while upgrading a server (remotely) to 4.19.78 (longterm).
*Bug 205181* https://bugzilla.kernel.org/show_bug.cgi?id=205181 - kernel panic when accessing btrfs root device with f2fs in kernel https://bugzilla.kernel.org/show_bug.cgi?id=205181
Andrew.