I think Sinner has a pretty poor record in fifth sets, too. The only match he has won in a fifth set in his four slam titles is the one against Medvedev in his first-ever slam final, and it helped that Meddy was out on his feet by the end. I remember the commentators mentioning it when Dimitrov was two sets up as reason to think that Sinner would face a bigger test overcoming that deficit than would, say, Alcaraz or Djokovic.
By contrast, Sinner lost in five sets at RG 2025, WI 2024, RG 2024, so even in the year and a half that he's been the tour's dominant player, his record in fifth sets is 1-3. So, I do think Sinner's mental toughness is still questionable, although like Federer his resilience, by which I mean ability to bounce back from tough losses, seems very good.
In addition, Sinner lost in a fifth set at USO 2023, RG 2023, AO 2023 [he did win a fifth set at AO 2023, too, though], USO 2022 [he actually won two fifth sets at that event], and WI 2022, so I think his fifth-set record in the last four years is 4-8, and in the last three years it is 2-6.
In 2021, though, he won two fifth sets (v Herbert at RG and v Monfils at the USO) and only lost one (v Shapovalov at the AO). In 2020, he lost a fifth set at the USO v Khachanov. I don't think he played any in 2019.
So, unless I'm missing some from Davis Cup or some other team tournament, I think his career record in fifth sets is won six, lost 10.
(The win against Dimitrov doesn't count as a fifth-set victory even if one were to count it as winning a match from two sets down).