The mdsync process keeps Mac from sleeping

I noticed that the process named mdsync (part of Spotlight) only runs after I connect a Time Machine disk to my MBP. It can run for a very long time (hour or more). When it runs it blocks the computer from sleeping. The issue with this is that it essentially keeps my Mac from sleeping during the day.

My MPB has three Time Machine targets. Two are local SSDs connected via USB. The third is a NAS connected via WiFi. The USB connected drives are not disconnecting and reconnecting during the day, but the NAS storage is. Every three hours Time Machine connects to the NAS, mounts the disk image and backups up my data. Then the networked drive stays connected for a very long time as mdsync (and other associated Spotlight) components run for a very long time. This leads the case of the Mac not going to sleep for the entire day - even if I go out for a few hours and stop using the computer it is always running.

Is this normal behavior mdsync? Are other people with similar setups also seeing their Macs not sleeping?