Hi Matti,
this is a known limitation, and it’s not related to ScrollView in any way.
Rendering of Text on the screen requires a given amount of memory. That amount depends on many factors, like the font you use, font size and even what the characters are (drawing a dot is easier than drawing 竜, for example). And not only that… the amount which triggers the text to disappear will vary on different target platforms, OS versions and even single devices.
So if you’re asking if there’s a maximum length for content, the answer can be both “yes” and “no”, depending on how you look at it. There certainly is a limit, but it’s not a set number of characters.
Your best bet is to split your text content into pieces and stick them back together with multiple Text elements in a StackPanel or WrapPanel, or any other fashion that works for your app design.
Hope this helps!