I can confirm I noticed such a behavior - it does not happen always, but I was able to relicate more easily with some messages marked and moved in one step by drag&drop.... just moving messages one by one I was not able to see this behavior yet.
Looks like there is an issue here - but needs some more tests/analyzing under what circumstanced the read-status gets resetted/changed back to unread.

I just wanted to chime in here to confirm I was able to reproduce this in testing here. I did find the unread status is seen first in webmail, then syncs back to the Outlook client which I found interesting. We'll get this escalated for you, thanks for the report!

Also, while escalating this case I tested it under current, previous, and future builds and was able to find the issue was present in 8125 as well and so doesn't look to be a new issue at least!

I confirm that it is an old issue, but never reported erlier

I just wanted to follow up on this thread to let you know we were able to get this resolved under our internal builds (within testing currently) and you'll see this fix in a future release of SmarterMail.

Good! I'm really excited to see a new release (soon, I hope)