sudo + private-etc throws 'unmount: Invalid argument'

[glitsj16]

I noticed two warnings when using private-etc with sudo firejail foo that I don't understand:

/etc/group: unmount: Invalid argument
/etc/passwd: unmount: Invalid argument

Just trying to grok the code and wondering whether other people have seen this.

$ firejail --noprofile --private-etc=foo echo "bar"
firejail version 0.9.73

Parent pid 46952, child pid 46953
Private /etc installed in 35.69 ms
Base filesystem installed in 0.10 ms
Child process initialized in 65.48 ms
bar

$ sudo firejail --noprofile --private-etc=foo echo "bar"
firejail version 0.9.73

Parent pid 47058, child pid 47059
The new log directory is /proc/47059/root/var/log
/etc/group: unmount: Invalid argument
/etc/passwd: unmount: Invalid argument
Private /etc installed in 35.89 ms
Base filesystem installed in 0.08 ms
Child process initialized in 51.86 ms
bar

Here's the relevant code AFAICT:

firejail/src/firejail/sandbox.c

Lines 1049 to 1052 in dbf206d

	if (umount2("/etc/group", MNT_DETACH) == -1)
	fprintf(stderr, "/etc/group: unmount: %s\n", strerror(errno));
	if (umount2("/etc/passwd", MNT_DETACH) == -1)
	fprintf(stderr, "/etc/passwd: unmount: %s\n", strerror(errno));

Nothing seems to be broken, but the output is confusing (me). Would be thankful for some pointers on how to interpret this part of Firejail's code.

[glitsj16]

UPDATE

Did some more digging. Still unclear why this happens. The output doesn't show when using the --debug option, nor does it happen on my Ubuntu 22.04 LTS box... Adding it to my 'unexplained harmless glitches' file.