What is this pipe? It leaks when the sink runs
Poly is flexible, I haven’t seen it used as a drain like abs that is rigid.
With a septic system in winter it could be your field is flooded (standing water where the field is?).
Another cause is a partial plug up below this point it’s not always a failed system heavy rain fall and a flooded drainage field the water takes longer to dissipate into the ground.
That could have been a condensate drain for the AC system but not well done. I would cap it using a rubber cap. Fernco makes them.
Cut off pipe square slide cap on and tighten the hose clamp 5$ fix for now.
There may be other ways but a fernco end cap would be my choice in this case.