When converting existing logical volume to thin pool logical volume, LVM needs to open the volume temporarily for it to be initialized with zeroes at its start. However, this initialization step caused WATCH udev rule to trigger. The WATCH udev rule is set for all top-level logical volumes to update udev database records if device content changes - the watch udev rule triggers on each close of the device once if it was open for writing before. This is exactly the case of opening the volume for thin-pool initialization. All udev rules are reevaluated based on the WATCH rule which causes subsequent scanning of the device for changes. At the same time, LVM may try to close the device, ending up with an error to be issued since LVM is unable to remove open device.
There was an error message issued while converting to thin pool volume: "device-mapper: remove ioctl on failed: Device or resource busy".
LVM now uses proper flags for temporary volumes which are used during conversions as intermediate step. These flags direct udev to avoid setting WATCH rule or initiate any scanning on such devices until they're properly initialized.
There's no message about device-mapper device removal issued anymore during logical volume conversion to thin pool logical volume.