Form validation replaces inserted values with special characters replaced by html entities

I have a simple contact form where fields are validated with patterns.
When trying to send the form and validation of the fields fails, the values filled in the form fields are replaced by an "escaped" version (with html-entities) - means eg.:
"Thomas Test" becomes ""Thomas Test""
"[email protected]" becomes "test@test.com"

Could anybody help me how and where to fix that?
Thx in advance!

Great question, well explained.

Have you tried searching the Form plugin issues on Github? I'm not saying that's where the answer is, only that it seems like a good place to start. There are a few issues that seem similar. Please tell us if that helps.