which also makes copies of the constraints which are completely independent from the original constraints on the left-hand side - delete the stupid new right-hand side constraints and slowly, painfully constrain the right-hand side copies to the original left-hand side elements
I feel like this is not how it works for me. It does copy the constraints and automatically create symmetry between any points across the selected center line, but if you're making a mirror why is that not what you want?
Also I'm unsure about the behavior in older versions of freecad, the constraint behavior of this feature is new in 1.0
Here's how it works for me: https://vimeo.com/1019673822
Sorry I forgot to click record cursor. But hopefully that's clear enough.