I'd like to see stats and truly know what accounts for these drops.
The only way to get some sort of idea is with A/B testing, which is really hard to do during a pandemic. However, there is a body of evidence to suggest that being in daycare around 3+ helps with later success in schools: https://www.apa.org/news/press/releases/2023/06/quality-child-care-science-math
Also, you seem to point out to more time for parents to be able to read to kids if everyone works from home, but that may not be the case. WFH does seem to indicate that people work longer hours in total. They may also not be able to spend as much break time during the day to. Do things like read to their kids or otherwise mentally simulate them.