I also bought the Paderno blue steel 40X30cm, I seasoned with olive oil as recommended. It stuck on the 1st bake, but only slightly in the corners, was able to lift out -no damage to either the pie or the pan. Now I ordered 2 of the detroit pans (10X14 & 8X10) *unseasoned* as seasoned was out of stock mid 2020. On one I used vegetable shortening (mixture of soy oil, and palm oil and hydrogenated for thickness) I think I did 5 VERY THIN coats at 445F (just under the smoke point of soy oil) -this one went immediately into service, same sticking story as the Paderno blue steel, only once, no sticking after that.
The other I did that "scientific" flax seed oil seasoning, finding the least refined oil I could -it even requires refrigeration after opening.
This pan sat in an airtight box for about 4 months in a dry closet, then I took it out for a bake as it's size was 8X10 for that experimental bake.
It seemed to spot up as you mentioned, as I looked at it thru a magnifying glass, it seemed to be developing SURFACE RUST UNDER THE SEASONED COATING! -ergo sticking it would.
I took it and using hot water, NO SOAP, and a plastic scrubber, took out all the surface rust.
Then I used the vegetable shortening 5 VERY THIN coats at 445F (just under the smoke point of soy oil).
to make a long story short, IT NEVER STUCK EVEN ONCE. Ever!
I think seasoning oils resistability to moisture is the key to a killer seasoning, as the palm oil in the vegetable shortening is a high smoke point oil with the fatty acids that are easily solidified, nearer to..... say BEESWAX, and resist moisture ergo tomato sauce, steam, and of course oily cheeses.