So, we had some stir-fry burn up pretty badly in our trusted cast-iron skillet. Upon cleaning the skillet some of the coating came off with the burned food. I made a first attempt of re-seasoning it with avocado oil and putting it upside down in the oven for an hour at 375°F. The result was mediocre, there was some coating in the stripped areas but it didn't look like the rest of the surface, and you could see some bare metal shining through. Sure enough, food was sticking again to the stripped area.
My wife re-seasoned it again, this time following slightly different instructions that were on the internet: using olive oil and baking it for two hours right-side-up at 450°F. At the end of the process the skillet definitely had a new hard coating that was smooth and not sticky to the touch. However, there was an anomaly in the middle (probably around the same spot that got stripped originally, but we can't be sure). There was a raised ridge around the area, but the area inside the ridge still did appear to have a proper coating:
So, my question is: what went wrong here and how does it affect the usability of the skillet? Do we need to strip it and re-season it yet again? Can we ignore this anomaly? Is there anything we can do to prevent this sort of thing from happening?