Heard from Mathworks support.
Licence checkout must occur before the FIRST initiation of a parpool.
"If you start MATLAB and immediately check out the toolbox license you need BEFORE the first time you try to open a 'parpool', then the workers will inherit the license. If you have initiated a 'parpool' at any point (even if you subsequently delete it) before checking out the license, then the parpool workers will not properly inherit the license until you restart MATLAB. In the event issues still persist and you are using MacOS, please increase the ulimit for file handles" ...
See Walter's comment above for more info on increasing ulimit on MacOS.