Why do OBJ meshes sometimes break when I import 2 OBJ meshes into one scene?

[Samsam2019]

So 2 OBJ files exported from Maya then put together in a scene cause one mesh to break apart as in every face has is its own object? 

This doesn't really happen with FBX exports but I notice it does with OBJ's?

[hamsterHamster]

With objExport.mll v.6.5 @ M2019, I could not replicate your scenario; tried various export options/combos. Yes, I've dealt with those split meshes before, and my guess regarding such topology is, it could be related to Normals | Smoothing settings upon export, as this is a way of making edges hard - by splitting the geo where the hard edges supposed to be.

Overall, my experiment always produced meshes with geo my Maya doesn't handle well and crash. E.g. selecting a face offsets the pivot to an unpredictable location, then transforming the selected face actually transforms other unpredictable elements at best, if not crashing. I've also dealt with such kind of geo, the Mesh>Cleanup doesn't do a thing, and I could not find any other safe fix than rebuilding the geo from scratch, a complete bummer.

VERDICT: unless you have updated plugin version (see plugin's Info in Windows>Settings>PluginManager list), avoid OBJ format completely, stick to FBX if you absolutely have to use non-Maya format.

[Samsam2019]

Thank you, I think it is because of the smoothing meshes.