Masks are primarily to protect others, so the concern is not the mask-wearer catching it, but rather the mask-wearer spreading it.

There are two concerns related to post-immunity spread: First, the accuracy of tests, and second, the ability to spread despite immunity.

I believe that some studies have shown you may still be able to transmit the disease if exposed, even though you yourself are immune.

I don't have sources handy, but that is a starting point for your own research.