I think this is either a bug or an undocumented limitation. In my game, I list everyone in the credits that has contributed financially by donating one way or another on my Twitch stream. Surprisingly, this has created a rather large list to include in the credits. I store the names in a lua module because I call them both on the main menu and in the ending.
I just noticed that the names get cut off at a certain point (they are all placed in one single gui text node). I manually pasted the names into a node in the editor to verify, and got the same behaviour.
I can split this into multiple text nodes no doubt, but thought I’d report it anyway.
For scrolling credits, what I did for Interrogation is that I split the credits text at every \n and then use a RichText instance (you can use a simple GUI node) for each line. Then, as the credits roll scrolls, I only create the lines that are about to come on screen and delete the ones that go off screen. That way you can stay within the total character limit for GUI text nodes.