While Synology is good, there is one thing they are infamous for… forgetting (dropping) share permissions randomly.
I never had this kind of problem on any other devices using smb share.
The mean problem comes (for me) from the NVidia Shield. I had to reconnect lan mounted shares after rebooting it
It’s pretty much a ‘Oh well, that’s Synology’ quirk. One can only hope it’s been resolved in DSM 6.1