Not sure why, but I have a similar one that has been in process since 3/12/21. Spoke to IRS today but was told it is still processing and they are way behind because of all the verifications required on the 2020 return.
Did your client file MFJ in 2019? Mine filed MFS in 2019 and MFJ in 2020 so I think that might be part of the delay, but it's just a guess. IRS could not give me specific details on the client's return and just read off a list of excuses for the delay.