If you’re not using some sort of Domain mapping, then the use of the same mount by two different sharing services with different uids is going to break ownership. Doesn’t matter if it’s Synology or anything else.

NFSv4 domain mapping solves this by having the same domain configured in client and server. That’s probably your simplest option. From memory, I do believe Synology DOES set uid for whichever user is authenticated via SMB and NFS though, so are you using two different users for these mounts by chance?

If you don’t want to bother to setup LDAP or domain mapping, then just use SMB and that should solve the problem.