Comment 23 for bug 1923670

After figuring out the correct upstream repo for the ubuntu kernel repo (not https://git.launchpad.net/ubuntu/+source/linux but https://git.launchpad.net/~ubuntu-kernel/ubuntu/+source/linux/+git/focal) I replicated your command

$ git log --grep 'cifs' Ubuntu-5.4.0-70.78..Ubuntu-5.4.0-71.79

and the most recent commit (f28b34becc655b6139216a72d12378001faa5784)
caught my attention. Indeed it was reverted upstream 2 days ago [1]. I think those are the related discussions on linux-kernel [2] and debian-user [3]. The last one describes the same scenario.

1: https://cdn.kernel.org/pub/linux/kernel/v5.x/ChangeLog-5.4.112

2: https://www.mail-archive.com/search?<email address hidden>&q=subject:%22Re%5C%3A+%5C%5BEXTERNAL%5C%5D+Re%5C%3A+%5C%5BPATCH+4.19+013%5C%2F247%5C%5D+cifs%5C%3A+Set+CIFS_MOUNT_USE_PREFIX_PATH+flag+on+setting+cifs_sb%5C-%3Eprepath.%22&o=newest

3: https://lists.debian.org/debian-user/2021/04/msg00062.html