How can I absorb Toriel's SOUL?

In my first playthrough of Undertale, after killing Toriel, instead of her SOUL cracking and shattering, it was merged with mine. It happened only on my first playthrough (neutral).

Here’s what I know for sure:

• I got hit by flowey. I didn’t evade his “friendliness pallets”.
• I killed the dummy.
• I took all the candies I could from the jar (tipping it over)
• I bought one of the cheaper kind of spider products.
• I did not take the Toy Knife, I fought Toriel with my stick.
• I never fled from a battle.
• I “killed” Napstablook.
• I picked Cinnamon when asked.
• I never called Toriel back during my stay at the RUINs.
• I tried to talk to Toriel once, spare her once, then proceeded to attack her for the rest of the battle.
• I didn’t reach 2HP (her attacks didn’t evade me)
• I didn’t die at any point in the RUINs.

What I don’t remember for sure:

• Did I get the pie? I didn’t go to sleep. But I don’t remember if I had gotten it nevertheless.
• If I had gotten the pie, did I eat it during her battle?
• Exactly which monsters I’ve killed during my stay at the RUINS.

I don’t have a savefile of the game, I only discovered that it was abnormal after I’ve done a genocide run and decided to wipe the game files for a true pacifist run.

I’ve made neutral, pacifist, genocide, pacifist runs, in that order. I obviously didn’t see it in the pacifist, and I attributed her SOUL shattering in the genocide run to my cruelty (I hit her so hard, her SOUL couldn’t recover).

Have anyone experienced this? Do you know what can possibly be done to attempt and reproduce this? I’ll answer any clarification requests in the comments, this issue is driving me crazy!