The course makers seem to want to avoid dealing with mutations at an early stage, which is fair enough, but this does seem a little unnatural. This could be avoided if they used "heddiw" instead of "ddoe". Worth suggesting?
I'm a second-language speaker, and have been teaching Welsh to adults for 18 years. Things like "Dw i wedi cael bath ddoe" are among the incorrect examples we use to explain the difference between the perfect and simple past tenses.