Also, the report says that both red and blue states had both overcounts and undercounts, which is a different picture than "over counts in blue states". It is accurate that more blue states had overcounts and more red states had undercounts.
I realise your question is rhetorical, but the report does offer a possible answer: "The 2020 Census faced many challenges such as conducting fieldwork during the COVID-19 pandemic. Other challenges to the 2020 Census included con- troversy around a proposed citizenship question and changes in the duration of the Nonresponse Followup operation." It's plausible that those "challenges" could have been experienced unevenly by state, particularly with how politicised the pandemic and its mitigations were in the US.