[UPDATE: The separate releases issue described below is gone, possibly after a reboot]
[Update 2: This is still an issue]

The dock treats all releases of Edge (Canary, Dev, Beta and the production release) as the same file. They are all msedge.exe files but they are all different and in different locations.

So if you add Edge Beta (or dev, or canary) to the dock, the release version always opens.

Right click on the icon and choose 'Open' and the correct application opens.

These are basically Microsoft issues - i.e., they are caused by Edge not informing third party apps which version is meant to open when the same .exe name is used.