Helping you further requires more knowledge of the code and more access to your site, so I’ll leave it to @takayukister. You might want to try opening an issue on https://github.com/takayukister/contact-form-7/issues.
Tiny suggestion: when you post formatted message or code, use the Code block, instead of simply pasting. Do the same on GitHub, if you start an issue there. It makes things a lot clearer.
Final thought from me: I don’t have this issue on any of my sites, and I see relatively few reports about it from others, so you might want to try deactivating plugins and using a standard theme. Deactivate anything related to contact forms first, but if no luck, keep going.
Ideally, clone the live site, and use the copy, but if not, sacrificing a few minutes of site availability may be worth finding the solution and being able to keep your site up to date.