I have not run across this issue and I'm not 100% sure, but I think the IRS uses the 2020 return to determine which parent is eligible to receive the stimulus and future credits.  This is based on an article I found in Forbes but I would defer to someone else who knows for certain:

https://www.forbes.com/sites/heatherlocus/2021/03/31/how-divorced-families-can-maximize-their-relief...