Hello together,
I can confirm this issue, too.
After upgrading SmarterMail to 14.3.5752 there are subject lines at some mails where the spaces are replaced by underscores. The mails are from outside my internal mail domain, where I can see the underscores in the subject lines. But not all mails from outside my internal mail domain are showing this issue. I'm only working with the webmail interface.
Kind Regards