You mentioned that "...I've tried converting the FBX to OBJ+MTL..." - using what ? I never import an FBX into Poser directly - I always* import it into Blender (I've stuck with 2.79b as I don't like 2.8) , do whatever's necessary there (which would include in this case Object > Join), and export as an OBJ. Then I only have to deal with Poser's OBJ import idiosyncracies. I've never* seen the problem you mention... but see footnote. So sorry, not much help.

*full disclosure: I have to say that I've only ever touched an FBX once or twice :o)

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The 3Dcheapskate* occasionally posts sensible stuff. Usually by accident.
And it usually uses Poser 11, with units set to inches. Except when it's using Poser 6 or PP2014, or when its units are set to PNU.

*also available in ShareCG, DAZ, and HiveWire3D flavours (the DeviantArt and CGBytes flavour have been discontinued).