I have done builds on fresh multiple machines now, including an AWS EC2 instance, and over the past few months I consistently get this failure on a fresh clone of the cesium-for-unreal/ue5-main branch.
When running ./RunUAT.sh BuildPlugin -Plugin="$CESIUM_FOR_UNREAL_DIR/CesiumForUnreal.uplugin" -Package="$CESIUM_FOR_UNREAL_DIR/../packages/CesiumForUnreal" -CreateSubFolder -TargetPlatforms=Linux
it fails on every file with a size mismatch error
Size mismatch /home/ubuntu/develop/cesium-unreal/extern/cesium-native/extern/KTX-Software/other_lib/mac/Debug/SDL2.framework/Versions/A/Frameworks/hidapi.framework/hidapi = 23 to /home/ubuntu/develop/packages/CesiumForUnreal/HostProject/Plugins/CesiumForUnreal/extern/cesium-native/extern/KTX-Software/other_lib/mac/Debug/SDL2.framework/Versions/A/Frameworks/hidapi.framework/hidapi = 130
When i execute this on my corporate VPN, which is sometimes subject to packet loss and significant latency/jitter I even get a timestamps are not valid (because it took longer than 2 seconds for the copy of the files to complete).
The resolution I have used is to modify the source code of the UE5 AutomationTool Utils so that it does not do any of these file copy verify checks. Primarily by commenting out these lines and making sure Retry is never set to True
Appreciate the help/insight and a better resolution to my hack going forward!