Problem with model after 3ds max export?

Question

3D

Good day!
There is a blank made in 3ds max, an example below:

A little about building a model
Due to poor command of the program, the part was built from a pipe, adding elements and cutting out gaps using boolean operations. When exporting to Stl format, a problem immediately arose, the part was transferred along with those elements with which I made holes. After that, I found a checkbox to export only the selected part, and everything seemed to work out. But...

Having saved the inverted part in gcode, to print fewer supports, I started printing ... and was surprised to see that the support was still printed in the center of the part, I decided to wait and print the part to the end. As a result, for some reason, there was filling on the finished part. This screen is from the preview of the model, and the filling is also there ... although there is an empty space on the part itself. I assume that this is still due to my modeling with boolean operations ... tell me who knows what could be the matter? Can you try to save the model first not in Stl format?