Steps to reproduce:
1. create pipeline with script for windows node

node('windows') {
  ws('D:\\workspaces\\ws') {
    ws('D:\\workspaces/ws') {
    }
  }
}

2. run pipeline

Expected output:

[Pipeline] ws
Running in D:\workspaces\ws
[Pipeline] {
[Pipeline] ws
Running in D:\workspaces\ws@2

Actual output:

[Pipeline] ws
Running in D:\workspaces\ws
[Pipeline] {
[Pipeline] ws
Running in D:\workspaces/ws

The unexpected behavior occurs because FilePath does not normalize file separators and the FilePath is used to associate the lock in the inUse-Map of hudson.slaves.WorkspaceList.

It works for relative paths because FilePath normalizes file separators in this case (due to -JENKINS-13649-), see FilePath#resolvePathIfRelative.