Perhaps it isn't as simple.  For example, you didn't mention that non-resident aliens and all dual status aliens are not eligible for the RRC.  You also didn't mention other oddities that would make RRC not eligible.  The IRS has the records and holds the reasons for the adjustment.

2018 and 2019 AGI's affected the EIP which in turn affected the balance of RRC on the 2020 return.   As originally posted, the info served as usable clues for the puzzle and of course would not be moot in the puzzle solving process. 

Ultimately, only the IRS has the answer for the reason of the adjustment.  So far, the information ($3518 claimed RRC) tends to imply two adults with a child .  Perhaps there are reasons for not revealing all the details.  I just worked with whatever was given and used them as clues and tried to eliminate the obvious.  

I come here just for kudos and puzzle solving.  The rest is between the Clients/CPA and the IRS.