Windows 10 SMB to Samba on Debian file modified time changes

I'm experiencing a weird problem where a file modified time changes after some time.

Here is what happens:

1. C# DotNetCore app running on Win10.
2. Opens file over SMB on Debian (Proxmox) with ZFS and Samba.
3. App modifies file, and records size and modified timestamp for later use.
4. Time passes.
5. App processes same file, compares modified timestamp with stored timestamp, and it changed.
6. The file did not change, only the timestamp reported changed.

If I had to guess; this looks like some kind of SMB caching issue where the timestamp reported over SMB does not match the actual file timestamp, and opening the same file later then reports a different timestamp.

I cannot reproduce this on demand, but I can observe the behavior by looping and reading the modified time, waiting, re-reading, repeat, and after some time, sometimes, the reported modified changes from previous values.

How can I get a more deterministic timestamp behavior?