With the best will in the world, though, I struggle to see how she could have continued as an actress … all her roles would be scrutinised, cries of she only got the role because of who she is and then they would have to be agreed by committee to make sure there was nothing inappropriate that could cause the RF embarrassment.I don't know, this is speculation.
I could imagine that it was a more due to the aim of slimming down the monarchy.
TQ offered (advised?) MM to keep working as an actress and why would she do that? Certainly not if she had issues with MM, her professor or her appearance (given that actresses are in the public domain and everyone would have known that MM is married to PH).
It was also clear that Charles was keen to slim down the monarchy. Maybe they would have been happy for MM to carry on working and for PH to do whatever he wants, now that he was no longer needed as a spare as PW had three children.
This is also speculation, but maybe PH and MM were offended by that and then linked the "perceived snub" to race
Good in theory, impossible in reality but nice PR.