I have a fix that works for me, but these forums are terrible and are not letting me reply with the actual details.
I created a script that changes some SMB config and the issue went away for me on DSM 6.2.3.
Here's a google doc with instructions:
https://docs.google.com/document/d/1...it?usp=sharing
I reached out to Synology and they're only interested in troubleshooting by jumping through hoops so I give up on working with them.