Operating system: Windows-10-10.0.18362-SP0 64 Bits
Graphics card: GeForce GTX 1070/PCIe/SSE2 NVIDIA Corporation 4.5.0 NVIDIA 451.67
Broken: version: 2.90.0, branch: master, commit date: 2020-08-31 11:26, hash: rB0330d1af29c0
Worked: (newest version of Blender that worked as expected)
Short description of error
So if you weight paint with x-ray activated you should be influencing vertices that are in the background too.