Historical dividend/cap gain data is incorrect. Dividends are accounted for but cap gains are not. This distorts the adjusted price.
Michael Loudiana commented
I have the same problem with mutual fund adjusted close values. Prior to January 2021, mutual funds were adjusted for capital gains and dividends. Some time in January 2021, Yahoo stopped adjusting for capital gains. Yahoo claims they are using CRSP documentation and the documentation says "Split events always include stock splits, stock dividends, and other distributions" Capital gain distributions (which are "other distributions") should be included so adjusted close reflects total return.
Harold Williams commented
This problem started sometime in 2021. Prior to that adjusted price data was correct.