Code in seq_render_mask will effectively do BKE_mask_evaluate(mask_temp, mask->sfra + (cfra - fra_offset), true) where fra_offset is zero for absolute and seq->start for relative.

If we really want the scene's current frame (as advertised) if Mask Time is set to Absolute (effectively ignoring the Mask Settings start/end) we need to change the fra_offset from zero to mask->sfra.

Also BKE_animsys_evaluate_animdata should take mask->sfra into account as well (otherwise mask animation [points] and other animation [e.g. opacity] will run out of sync)