That's because it is a "Nyquist Plug-in" effect.
See here for more information about Nyquist plug-ins:
Nyquist plug-ins do not currently support preview.
The Debug button serves two functions, it allows users that write their own
plug-ins to see any error messages that may be produced due to programming
errors. It has also been used in some plug-ins as a method for outputting
text that can easily be copied and pasted. (Windows does not support copying
text from the normal text output).
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