So I have some more info on this:
My application had a hierarchical block, inside which I had a block (called "chopper") that was setting the set_output_multiple.
This block was the source of the warning:
gr::log :WARN: flat_flowgraph - Block (chopper0) max output buffer set to 6656 instead of requested 8192
I don't know why this warning was given (I suspect the size of the output buffer is forced to be a multiple of the set_output_multiple and less than 8192?),
but when I reprogrammed the "chopper" block and removed the set_output_multiple from it,
neither do I get the warning nor does the application crash (when i run it with controlport on).