Well, having said the above, I think the frame must be before '90.
And here comes something curious: the rear drop outs. They are very vertical and without rack threads, something unusual in those years even in the high range. They look like road bikes from those years
There is another detail that catches my attention: the absence of "dents" or "chafed" (I don't really know how to explain it in English) on the inside of the chain stays. It was very common at that time to be able to give clarity to the wide tires because also when welding with fittings the tubes come out in a predetermined direction, you cannot vary it, so you need that "flat" or make the chain stays very long.
In fact your chain stays seem long, greater than 44 cm center/center. It would be curious if you could measure them.
But there are more curious things: the frame has good quality details, but the cantilever brake supports DO NOT. It is a simple piece even for the 80s
The rear brake cable routing inside the horizontal tube is finished off in its holes with the classic road bicycle decoration.
The geometry is evidently very "square"
The bottom bracket in Italian (36 x 24) was little used in MTB. It is a sport made in the USA and they immediately applied the 1.37 x 24 tpi standard. Furthermore, the system is much more effective by threading the right cup backwards, preventing it from loosening with pedaling (quite the opposite) and therefore worked better with the MTB. It's true that some brands also used it at first, but it didn't last long.
My opinion: It is a handmade bicycle made with road fittings, road tubes and road and cyclocross accessories. Someone had some extra tubes and took advantage of them or a client asked them that way.
It's strange...yeah
It's unique...no. I know of at least one frame made with road tubes. I imagine there will be more
Just because the bottom bracket is Italian does not mean that the craftsman was. We could rule out the English, Americans and French (when the 35 x 1 metric system disappeared, the French "embraced" 1.37 x 24 in a general way) but in the rest of Europe they used English pitch and Italian pitch interchangeably to manufacture.
So it could be Italian, Spanish, Belgian, German...especially if it is someone who uses road equipment (the bottom bracket fitting is very significant)
Nice frame