It seems that the FBI seized their servers and found the private keys there.
If you wanted to know if there was a flaw in Bitcoin software or the protocol, there is no evidence of that.
https://krebsonsecurity.com/2021/05/darkside-ransomware-gang-quits-after-servers-bitcoin-stash-seized/
Edit:
The latest news indicates that ransom's bitcoins were deposited in a California based custodial address (an exchange or wallet provider) and the FBI seized it with a DOJ warrant.
So (astonishingly) it seems that the hackers lost their bitcoins because they used a custodial wallet (U.S. based) instead of one they had the private keys for.
The transactions flow also seems very easy to track: it is possible to do it by anyone even without knowing the ransom's address, knowing only its amount, as shown here:
https://blog.wolfram.com/2021/05/25/sleuthing-darkside-crypto-ransom-payments-with-the-wolfram-language/