I don't think there's any explanation per se, they just proceeded as if it wasn't an issue or had resolved. Sometimes movie makers gloss over these details assuming the audience will come to their own conclusion. I figured, since his return to Downton was only temporary, it wouldn't be an issue short term. Long term would have probably been a no-go and if that had happened, I'm sure they would have mentioned or addressed the palsy he had suffered from at the end of the series. Good question!