Welcome to the forum aureliobk.
I've just done some quick tests and the hyphenator looks to be working fine with pasted text in Scribus 1.4.6 on Windows 10.
I copied the entire text of Frankenstein from a text editor, pasted it into a new text frame in a new document, selected menu "Extras -> Hyphenate Text", and the text is hyphenated as expected. It also worked correctly when I used right-click + "Get Text" instead of pasting.
Resizing the text frame shows that that the text is also rehyphenated as expected.
From what I can recall, this worked on OS X too. I didn't use it much but I can't remember having problems with it.
Having said all that, using "Extras -> Hyphenate Text" before pasting the text in shows that the text isn't hyphenated. If there's no text to hyphenate Scribus could be ignoring the command. Could this be what has happened?
You might need to supply some example text and detail the exact work-flow you're using.