First of all, let me make one thing clear:
I’m not doing this to annoy you. I’m not concerned with who is right or wrong, but that the quality is not accidentally degraded because it is exported incorrectly. 
Interestingly, you even mention it:
Exactly, you basically decide whether to do the conversion in advance in Fusion 360, which unfortunately isn’t exactly known for doing a good job of mesh conversion (although this is often due to incorrect settings), or you leave it to the professional: the slicer.
“STEP format is a kind of standard exchange format that is widely used in CAD software. Almost all CAD software can export models or assemblies expressed in STEP format. That means models in assembly can be combined and expressed easily in STEP files without accuracy loss.”
“The STEP file saves models without accuracy loss. The original STEP file is the vector model, when importing the STEP file, Bambu Studio converts models into triangular mesh automatically according to a tolerance that is small than the half of default arc fitting tolerance. That is, it is converted into a high-precision STL model in Bambu Studio. Therefore, more outline details of the model can be reserved, so that more G2/G3 arc move G-code can be generated when slicing.”
Edit: There is an extremely interesting thread on this topic. The user Olias did an outstanding job of summarizing this at the time, better than I ever could.
