I just typed "thumbs up" ~50x and was not able to reproduce the bug. But, as was pointed out in another thread somewhere, since I don't have 'Predictive Text' enabled maybe that has something to do w/it. So; I enabled 'Predictive Text' and there's the bug. It's consistently misspelling 'thumbs' with any number of different variations.
Disabling 'Predictive Text' seems to correct the bug; however, there must be something in the algorithm that's causing this that Apple does need to fix.