So I've heard people discuss this topic and was unable to find definite answers to it, so I decided to make a post myself.
Spanning singularity only targets ranged DPS players. Last night we did 40 pulls on Elisande where we got different amounts of singularities, but I'll be conservative and estimate we got 3 singularities per pull. This gives 150 singularities in total. We had 3 healers and 8 ranged dps.
If SS is able to target healers, then roughly every third SS should do so, statistically speaking. This means that about 50 of those 150 should have been on a healer. We had 0 singularities targetting our healers.
I kinda feel like this is statistically significant.
Bear in mind that we're not talking about the singularities that pops during phase change, these always target the person closest to the middle and can therefore target healers as well, we're only talking about the SSs that she casts during phases.