Suspicious red sand does not appear in the Mesa Ruins or anywhere else as far as I can see. It looks like it is not included in the structure, and the processor only looks for suspicious red sand, doesn't randomly replace regular red sand with suspicious (like is sometimes done for dirt, sand, gravel).

Hi, thanks for submitting this issue!
I have just checked the structure file and you're right. This must have been an oversight while creating the structure.

I have now placed Suspicious Red Sand in the Mesa Ruins and tested it. This fix will be available in the next version :)