Context
This recently came up internally when a customer tried to move from .sln to .slnx. Not all targets were included. The fix was to rename the existing after.{SolutionName}.sln.targets to after.{SolutionName}.slnx.targets.
Suggestion
For the smoother transition experience from .sln to the new solution format .slnx we should load after.{SolutionName}.sln.targets for .slnx too. We decided not to load after.{SolutionName}.slnx.targets for .slnx.