If I 'cd' in a ZFS snapshot I cannot mount it 'Device busy'

This code give the Device busy error:
zfs create ztemp/test_snap
cp /bin/c* /ztemp/test_snap
zfs snapshot ztemp/test_snap@first
mkdir /mnt/test_snap
cd /ztemp/test_snap/.zfs/snapshot/first/
cd
mount -t zfs -o ro ztemp/test_snap@first /mnt/test_snap
mount: ztemp/test_snap@first: Device busy

This code works:
zfs snapshot ztemp/test_snap@second
mount -t zfs -o ro ztemp/test_snap@second /mnt/test_snap
ls -l /mnt/test_snap/
total 367
-r-xr-xr-x  1 root  wheel   14600 Mar  1 11:33 cat*
-r-xr-xr-x  1 root  wheel    8920 Mar  1 11:33 chflags*
-r-xr-xr-x  1 root  wheel   20720 Mar  1 11:33 chio*
-r-xr-xr-x  1 root  wheel    9616 Mar  1 11:33 chmod*
-r-xr-xr-x  1 root  wheel   22496 Mar  1 11:33 cp*
-r-xr-xr-x  1 root  wheel  442056 Mar  1 11:33 csh*
umount /mnt/test_snap

Tested in 13.1-RELEASE-p3 and 13.2-BETA3

Regards
Maurizio