When you import an stl file into tinkercad you are then also relying on how well its import function works. Generally you cant expect to export to a different file type and then load that back in and have it behave as the original model with ANY 3d modelling software. The export to a different format pretty much loses some information, especially to a simple format like stl.
Tinkercad was founded by former Google engineer Kai Backman and his cofounder Mikko Mononen, with a goal to make 3D modeling, especially the design of physical items, accessible to the general public, and allow users to publish their designs under a Creative Commons license.[3] In 2011, the tinkercad.com website was launched as a web-based 3D modeling tool for WebGL-enabled browsers,[4] and in 2012 the company moved its headquarters to San Francisco.[5] By 2012, over 100,000 3D designs had been published by users.[5]
Rather than wait for that, try tinkercad:
Tinkercad Tinkercad Create 3D digital designs with online CADTinkercad is an easy-to-use 3D CAD design tool. Quickly turn your idea into a CAD model for a 3D printer with Tinkercad.