List of browsers and versions that work, but should be most modern browsers. If you don't permit port 80 access in the DSM firewall (and Internet firewall) then you should be protected from insecure access.
At a loss to know why it's not working for you then. You have also got HSTS enabled in Control Panel / Network / DSM Settings? It's the box labelled 'Automatically redirect HTTP connections to HTTPS...'.
It really sounds like there's a misconfiguration under the hood within the rewriting rules of DSM. You can open a support ticket with Synology and they will probably ask for remote access to the NAS to investigate it.
If you disable HSTS for one/all application URLs does it still go to Web Station home page?
so this one is still an issue for me.
the only browser that auto re-directs to HTTPS seems to be Google Chrome.
every other browser i've tested with lets you load the HTTP version if that's the URL you enter (Old Edge, New Edge, Opera, FIrefox).