From LINBIT's description, drbd_peerdevice_outofsync_bytes is what one node's bitmap says is still out of sync with one particular peer, while the UpToDate in linstor r l is the state of each node's own disk, so as far as I understand the two aren't measuring the same thing (the resource states are described at https://docs.xcp-ng.org/xostor#drbdlinstor-resource-state).
Your table has the 119 GiB on .10 and .14, the two Unused copies, so I'd be curious which peer those series point at; the labels on the metric should say. If I'm reading drbd-utils right, drbdsetup status xcp-volume-6a8544e8-f17f-49c8-b725-54c5e3213ae1 --verbose --statistics on xen01 also prints an out-of-sync figure per peer, but I haven't got an XOSTOR cluster to try it on, so treat that as a (not so educated) guess.
Whether a non-zero number between two secondaries means anything is beyond me; @Team-Storage will know.