After a careful Plex Dance as you described, and following 99% of what you say to do, I get the same result. The SRT is automatically added, appearing on the Pre-play screen, is automatically chosen and gets played with the movie in Plex for Mac. I’m very careful to follow the Plex naming conventions and to use the Plex Movie agent and Plex Movie scanner.
The 1% anomaly: maybe just how you phrased it, but below is not the normal procedure:
When you add a new video, you Scan Library Files which causes a metadata refresh on the new item. I’ve never heard of adding something and doing a Refresh Metadata only. Very happy you mentioned your Plex Dance, though. I forgot to try that 
