This seems quite similar to How to get rid of these outline artifacts in an Inkscape SVG trace? but I thought asking again would be helpful since it seems that at least the preview does not have the issue.
The source file is a 160x160 bitmap of the smiling face with halo emoji, Apple's version.
I imported this into Inkscape 0.92 (on Ubuntu 18.04) and used
Path --> Trace Bitmap on the imported raster image object. The preview and final result are this:
Notice how the preview looks perfectly fine, but the final result has these outline artifacts, interrupting the smooth and continuous color gradients. The thing is, the preview looks fine and has the smooth and continuous color gradients, unlike the final trace. And yes, the preview is actually of the final trace, I checked this by adjusting other settings and yes, the preview changes as it should.
Is this an Inkscape issue? Or perhaps a quick fix in inkscape to make the final trace look like the preview? I'm not sure, but it seems like I'm missing something obvious.
Thanks for the help!
Final trace result: link (please download first and open in your browser, the drive preview isn't accurate)