Based on your comment about how your setup doesn't support GL/CL sharing, I tried defining FLG_FOSPHOR_USE_CLGL_SHARING as 0 in private.h. Not sharing GL objects fixes the pixelated spectrum issue! No more cl_clear_queue_clear_buffers errors either, because the code never goes down that path anymore.
It seems to me there is a bug in the way the histogram buffer or texture is allocated when it is allocated through the GL code path. I'm not sure if this is quirk of my setup/GPU/driver, or if this is a general bug; I do have an older NVIDIA GPU and another machine on which I can try to reproduce what I'm seeing.
Final question, what are the main downsides to NOT using CL/GL sharing? Is there some extra copy/performance overhead?