Interestingly, it seems like the eGPU stuff is causing all sorts of issues all around. Very similar issues were reported using eGPUs and High Sierra:
https://forums.developer.apple.com/m.../264366#264366
The most interesting part (to me), is that where I noticed it thinks the external display is using the internal video card in the profiler, and in that thread, the same -- the external GPU seems to also be reported to be using the internal card(s).
Mind you, that thread is from late last year, but based upon Apple's docs, built-in eGPU support would be being added in to high sierra during this cycle.
I may be way off track, but it's an interesting coincidence if not related.