I can confirm that this works for me too. If I try to use saved credentials or if I enter credentials in the two-line form that only requests name and password, I can't play media via TVos; but if I type in fresh credentials immediately in the 4-line form that asks for user, password, port, WORKGROUP then I can play media from the SMB server.What worked was how I logged into the SMB server: previously I had just clicked Login, having User/Password/Port/WORKGROUP saved. Instead of doing that, I entered the log-in information manually: then, after going to the file with my movie on it, the orange "wait please" screen came up momentarily, but then the movie started playing.
And in the log file I submitted a couple days ago, it did appear (to a novice like me) that when VLC on TVos tries to log in using saved credentials it scans the keychain and fails to find anything. But there are other strange things, like the fact that when VLC shows me the local servers, there are two appearances of the Synology, one with the name in uppercase, and a second with the name in lowercase (which is how it is actually named), and on the iPhone, when I click on the server name (uppercase version) it shows 4 sets of saved credentials for that server, 2 using the server name and 2 using its IP address. So maybe somehow on the iPhone it just grabs the correct working credentials, but on TVos it somehow doesn't -- like because of a different sort order or something weird like that.
So I'm delighted that we now have a work-around that enables us to play media via VLC on TVos, and I wish there were something I could do to help identify the root cause of the failure.