It is unclear what caused the drastic drop in hash power, but several hypothesis stipluate that Bitmain may have reallocated more hash power towards AI-driven activities, or the company may have rented out most of its hash power to other companies. Furthermore, Bitmain may have moved most of its miners abroad and does not currently count this "exported" hash power as its own so it would not be affected by the imminent Chinese ban on crypto mining activities.
https://www.notebookcheck.net/Bitcoin-miner-maker-Bitmain-cuts-own-mining-hash-power-by-88.420369.0.html