When using the CreateBuildWithCustomContainer REST API I can get builds successfully created but they never seem to move to Deploying/Deployed state. (Upon inspection in multiplayer/server/builds). Active/Total/Standby values are all 0. I've tried changing the "Target standby" and/or "Maximum" from non-0 (they are initialized to 1) with no luck.
On the other hand if I bypass the REST api and use the web interface to add a build with what I can tell are the exact same configuration (same container image, same ports, regions, etc.) It will move into the deploying state.
I've read https://docs.microsoft.com/en-gb/gaming/playfab/features/multiplayer/servers/multiplayer-build-region-lifecycle and it says that "Initialized -> Deploying : StandBy servers have been configured to be > 0." As far as I can tell I meet that requirement with my EastUs region configuration above. (Target standby is > 0).