re-mentioning this topic because i think it got missed. it still seems to be an issue in the latest version of RW.
Confirmed. I thought it was Stacks at first, something I had done recently. But even in the plain vanilla Styled Text pages it works the same. Even if you try to enable it by hand with the contextual menu it reverts back to off immediately.
I also checked a few other versions:
RW v7.5.6 works
RW v8.0.3 works
RW v8.1.4 no spelling
RW v8.1.7 no spelling