This is a frequently asked question. To answer this question, let's slightly discuss how spherical reference works.
When calculating Huygens PSF with Spherical Reference, OpticStudio will first trace rays to image surface and then "go back" to exit pupil like below.
Then each ray interception point on the exit pupil will be considered as point source that radiate spherical waves and interfere on the image plane. This is called spherical reference.
On the other hand, when in planar reference, each ray is simply considered as a plane wave and still interfere on the image plane, which is same as spherical reference. The difference of the two methods will mainly be large when the Huygens PSF is supposed be large. OpticStudio will detect this by checking the distance from exit pupil to image surface and the expected size of the PSF.
Back to the discussion about which reference we should use. Usually, we can let OpticStudio decides automatically. However, there are some cases that the spherical reference is selected but doesn't work well. In these cases, usually the problem is that the exit pupil is at weird position. To diagnose, the first step is to check EXPP. If the exit pupil is not at a sense-making position, I suggest you manually force the reference back to Planar reference. In this kind of case, usually the exit pupil is at AFTER the image plane, where I don't think the spherical referenced Huygens PSF can be meaningful. Usually this happens when the a material is assigned to the image surface.