Hi coldforge_010 ,
Well that should bother you!
You have made a (possibly essential) mistake somewhere; maybe you have hidden part of your Mesh (ALT+H to get the hidden parts back), or part of the Mesh is actually missing, or you have 'double Vertices' (M > Merge > By Distance), or...?
The best thing to do, is to start from scratch.
I just did what you said, deleted the plane and started over, it worked, maybe I was just tired π΄Β
You can always watch the mesh modeling bootcamp, it's there where you'll learn all the rules of modeling: