This is plain wrong (and reported) the correct form is 'peidio â', 'to refrain from' or 'to not do'. And 'peidio' should not be mutated to 'beidio' without some reason. So 'peidio' on its own is correct, but it is mutated to 'beidio' in, for example 'rhaid i fi beidio ag anghofio' - 'I must not forget' where the mutation is triggered by the 'rhaid i...' pattern.
There are several instances of this, they have the mutated form (eg. ddysgu, gofio, goffi). I assume it's just to get you used to seeing the mutated form as well, but i agree that it'd be better to put it in context.
I have now pushed on through the tests to level 10 or so and the spurious mutations keep popping up. Annoying but not actually wrong.
The worst thing about the 'peidio â', though, is that it simply does not mean 'must not'. Peidio â - 'to refrain from'. In the context where it appeared the 'must' comes from 'rhaid i beidio â...'
It may have been corrected by now- they do seem good at reviewing and accepting corrections very quickly, which is good news on a beta such as this.