When displaying very large text in a variable on a form, the text may appear truncated.
The text is not actually truncated though, it is just the rendering of the text that is truncated.
You can select and copy the full text to the clipboard, but the display of the text looks cut off.

* The documentation mentions this limitation but does not give any precise limits (because the limit can be revised if necessary):

Note: Text variables and fields in forms are designed to display contents of a "reasonable" size. Beyond several tens of thousands of characters (depending on the system), only part of the text will be accessible in the form.

This Tech Tip attempts to address this question - What does "reasonable" size mean?

As of v15.4 and v16.0 -

On Windows a "reasonable" size is less than 100,000 characters
On MacOS a "reasonable" size if less than 1,000,000 characters.

This limitation only affects the rendering in the control, not the commands or the variable:
the variable or field always contains the complete text without truncation.
This only impacts large text and only display, not the datasource.