Do you known/guess whether the problem is the actual technical part, or everything surrounding it, which makes you appear as "sub-optimal cultural fit", e.g. person that is so nervous that they make everybody else feel uncomfortable too.
One time in particular I matched on every single technical requirement in the job post, only to be told that "experience with our tech stack isn't important as culture fit". I'm still not sure if he said that because he believed I didn't have the tech experience and wanted to make me feel good, or if he knew I had the experience but already knew he wasn't going to hire me.
Maybe next step is to figure out WHY you are perceived as bad culture fit?
Do you have any obvious issues like being very nervous in interviews, or underlying general issues such as anxiety, or just being a very quiet person?
Yes, all three. The nervousness and anxiety are especially heightened during interviews, but I don't experience either on the job. Does that make me not a culture fit?
Why don't you A/B test yourself? Take double dose of your favorite anxiolytic and dress more casually to next interview and see what happens. If you fail again then it's 31/31 and not a big loss.