The OS source and winpe.wim need to be at the same patch level.
Two scripts are provided for you to use (with caution - at your risk etc)
They should be run from an elevated PS Session
1. Create-WinPEv1803Media.txt- Runs WinPEGen.exe on an untouched winpe.wim (because WinPeGen won’t like it if you add updates first)
2. Add-CUToWinPE1803.txt - Add the CU to WinPE
"The examples in the scripts are using 1803, so change everything to match your source locations and versions. Make sure to check for any applicable servicing stack update that may need to be added."