Subject in mails with UTF and other characters

Good day,

we use Formmaker in the paid version and are very satisfied with it. It works great, it’s a great product. Unfortunately, after updating wordpress from version 5 to version 6, an error occurs. In the subject lines of mails to submitters and admins, there are number/letter combinations like “=? UTF -8 ?Q ? = C3 =”. It does not matter whether the subject is generated from the form title or entered manually. Form Maker is available in version 2.13.44. An upgrade is planned for summer. Until then it should still run on this version.
Is there a solution for this?

Thanks and best regards
Erik