I confirmed that 5.4.0-29 does not show the problem, and -31 does. Then I built -31 with these three patches reverted:
UBUNTU: SAUCE: overlayfs: fix shitfs special-casing
UBUNTU: SAUCE: overlayfs: use shiftfs hacks only with shiftfs as underlay
UBUNTU: SAUCE: overlayfs: allow with shiftfs as underlay
With those reverted I see the problem. It appears that this is a problem that the shiftfs hack for overlayfs was covering up, and then when the upstream behavior was restored whenever shiftfs is not the underlay it revealed the issue.
I confirmed that 5.4.0-29 does not show the problem, and -31 does. Then I built -31 with these three patches reverted:
UBUNTU: SAUCE: overlayfs: fix shitfs special-casing
UBUNTU: SAUCE: overlayfs: use shiftfs hacks only with shiftfs as underlay
UBUNTU: SAUCE: overlayfs: allow with shiftfs as underlay
With those reverted I see the problem. It appears that this is a problem that the shiftfs hack for overlayfs was covering up, and then when the upstream behavior was restored whenever shiftfs is not the underlay it revealed the issue.