Do your research. At peak Qubic had about 37% hashrate and performed what's called a selfish mining attack. All PoW coins are susceptible to this including Bitcoin. There were numerous 9 block reorgs (just below the 10 block confirmation requirement for finality) so there caused major delays in transaction timing but no finality was broken.

There was once a 18 block reorg which was clearly a major fuckup on Qubics part as come from beyond was in denial of it on X and was playing damage control. Off of memory about 100 or so transactions were invalidated and stuck in mempool for a week. Those could have been double spent against merchants thus the severity of the reorg. Only 10 and beyond block reorgs are damaging to finality though. Nobody reported double spends but it surely was possible at this one incident. Once again you don't need 51% for this attack, it's a selfish mining attack so Qubic got on a lucky streak, outmined the other pools in secret, then dumped their new longer chain on the network.

If they got 51% they could create true on-chain double spends, isolate miners and create permanant dominance over the chain. That attack is entirely different from the selfish mining attack we saw and would have resulted in permanent damage to the network.