See my detailed May 1st post Notes on Using an eGPU for Photoshop for Adobe Camera Raw Enhance Details.
May 6, 2019 update from Adobe:
There is indeed some confusion surrounding eGPUs. We've been working with Apple to try to understand in which situations eGPUs are currently supported and which ones they're not.
For normal Adobe Camera Raw usage, where we have more direct control over which GPU is used, you should find that eGPUs are supported in current versions of macOS, such as 10.14.4.
However, Enhance Details is an exception to this rule, because it relies on underlying OS-driven technologies where the use of eGPU is OS-determined, not app-determined. We originally tested Enhance Details with eGPUs back when the latest macOS was 10.13.x, and eGPUs definitely worked at that time to accelerate Enhance Details.
Unfortunately, eGPUs don't seem to be working reliably with Enhance Details under 10.14.4; we're working with Apple to try to get this resolved, but no timetable as yet.
MPG is glad to hear that the sitution is known and being worked on.