When the machine is rebooted and old thick snapshot merging is started, it may
take some time.
So there need to be something 'monitoring' progress of the merge and react
when merge is finished.
For this purpose the 'default' logic in lvm2 is 'lvmpolld' service - that
should be present in the system, so lvchange activation command can connect
lvmpolld service that would monitor progress of the merge. Once finished,
lvmpolld will basically --refresh VG and update metadata and drop
'empty/merged' snapshot.
There is also option to avoid using 'lvmpolld' and use the implicit
monitoring with 'lvchange --background' that would need to be used with
activation.
As such there is most likely no bug on lvm2 here - but likely some
configuration troubles.
Also note lvmetad is 'gone' for 2.03 version of lvm2 - so there should be
no connection with lvmetad udev rules.
