To reproduce:
1.Create new project and import attached "repro" package.
2.Notice that it contains 2 plugin dll - XInputInterface32 and XInputInterface64.
3.Check "XInputInterface64.dll" file settings in plugin inspector.
4.Notice in platform - windows standalone settings that both check boxes - x86 and x86_x64 are checked, where it should have been only x86_x64.

This is regression from 5.1.0f3 (4da421fac4b8)

Additional notes:
It also seems like sometimes platform selection doesn't import correctly too.