Surfacing - Maybe this has been answered before but I cannot find it.

Just a few cursory observations:

(1) If the notch is a feature, why not just delete or suppress the feature?

(2) Why use surfaces? That is the most complicated way to do this. I would probably just use a protrusion from the flat face adjacent to the notch. Have it extrude up to the flat wall with the two fillets and it should be okay.

(3) Whatever method you use, you should probably check the accuracy (File->Prepare->Model Properties->Accuracy). Set it to absolute, usually the default number is okay. This can make troubles with edges matching go away.

Of course, we don't know where the model is coming from. It could be a downloaded model. These often have odd surfaces.

Assuming this is a cylinder, use the convert in the import data doctor.  Pick both sides of the cylinder then convert to cylinder.

You do not even need to create the fill surface.