We were at MK on Christmas Day in 2019 and there was a brief phased closure around lunchtime that day. Lasted maybe about an hour or two around 12:30 if I recall correctly.
You will be alerted through the app if the park has started a phased closure. Since you will already have been in MK that day, you will not be blocked out of the park until the higher level of the phased closure so you will have some warning that it is going to happen. A simplified version goes like this- first they close the TTC parking lot, then they stop selling 1 day tickets and don't allow first time entries, then they stop allowing re-entries. If you see that the phased closure has started before you leave for dinner, you can always decide how much it is worth to miss your reservation- either 1) stay in MK skip the ADR and pay the $10pp penalty or 2) go to the ADR and risk not getting back in.
I would say the probability of having a phased closure in the evening is low on Christmas Day for a few reasons. People who got up early to open presents then go to MK are probably tired by the evening and want to leave the park. Christmas Day is exhausting for young kids since there is so much anticipation and excitement so not too many families will be able to hang that late at the parks. By the time you get back to MK from your 5:30 ADR it will be around 7:30. Not a likely time for a closure.
We will also be in MK on Christmas Day. We plan on leaving around 1 or so for a mid-day break and returning around 6pm. I'm not sweating a park closure as it is so unlikely to be blocked to re-entries for the entire night.