All of my movies/Shows (stored on Server) are experiencing this behavoir and 99.5% were recorded before I switched to MC18.
There have been some similar reports, but it is not universally true. It would probably help if we could get a handle on what is similar about systems that are experiencing this issue. File types, codecs, network setup... Anything specific to your system that could contribute to the trouble.
Are the files stored on the server directly accessible to the clients (via a mapped network drive or are they in the Library as UNC paths that resolve properly on the clients)?
I have a number of client/server setups, but for video use, I've always had the best experience making sure the answer to the above is YES. With that, and the relevant option enabled in MC (as it is by default), I'm seeing no similar issues.
I do not have any experience using MC in Client mode where it streams video to the clients. It can do this, I suppose, but the only times when I'd use this is if I'm away from home using my Laptop, and generally I'd just connect my VPN (and I'd really blame any problems on my relatively limited upstream bandwidth at home). I use MC in this way regularly for music, and that works fine, but I just don't use it for video that way.
My guess would be that, if it is related to the streaming functionality, it has more to do with MC18's reworked Gizmo video conversion engine than anything else.