Did you turn on the resources never deplete mod? It was added to the base game. The thing with specialized industry is they have two types of buildings. Raw materials extractors and refineries. What I think is happening is the ore in the ground is getting depleted. So your extractors are being replaced by refineries and the refineries are importing raw materials.
Unfortunately without turning on the no depletion mod ore and oil always eventually run out and all the specialized industry of those types becomes refineries that must import. If you don't have to worry about running out you can get imports very low.
Showing off my current city with tiny import numbers:
View attachment 272204
You can see I'm exporting every industry type in the picture I posted earlier in the thread of the same city. So its very possible to get low imports with every industry type.
I have definitely turned on the resource never depletes option in the mod section, that was a good move and saved using the mod from the workshop. So that has been on from the start of the map.
So my ore resource can't have ran out and demonstrating that was the large export of Ore and also oil.
For some reason I'm importing almost 700 or so Ore and yet exporting it in larger amounts, from the same section of industry. That can be seen from those two screen shots. I have included more screens below.
I did not know that specialised Ore industry can be broken down into two parts, extractors and refineries. That explains why the extractors "turn off" being coloured blue when the imports are selected and the refineries "turn on" being blue. When the exports are being selected then the extractors only I assume are showing blue.
Understanding the above makes me wonder why two Ore industries, next to each other, one generating Ore and exporting it and then the other importing it.....?
Option selected for unlimited Ores etc...
You can see in detail my Ore imports....
exports....
and, to a lesser extent, oil imports....
and exports....
You can see that the same factory which is exporting is also importing, in some cases.