Can you launch and run the MATLAB application(s) directly (not via MWorks) on the updated machines? It's possible that you need to re-activate MATLAB on those systems, and trying to launch the app is the quickest way to find out.
If the MATLAB application works as expected, then something must be amiss with the connection to MWorks. MWorks 0.9.dev (2019.04.24) should work with MATLAB R2016b or later, so R2019a and R2020b should both be fine. Once you've created the appropriate symlink to the MATLAB version you want to use, all that's left to do is restart MWClient, and things should work.
If you've created the symlink and restarted MWClient, and things still don't work, then I'm not sure where the problem lies. Maybe try restarting the Mac, in case there are lingering MATLAB processes running that are somehow messing things up?