What is this under babies tongue? Seperation/Gap/hole

Martha

Hello everyone, I noticed my 1 year old had a gap/hole under her tongue. I went through old photos and apparently she has had this for a long time.

Doctors office is closed due to COVID and Im driving myself insane over this.

I can’t find nothing on the internet!:(

I noticed this yesterday but this photo is from October 2020.

My other daughter does not have this.