File > New > File from Template… (command-N) [Resource] Asset Catalog -or- context (right mouse) menu > New File from Template… …

Double click the Hardware/Network Link Conditioner.prefPane to install it in System Preferences. It effects the network of the whole machine, not just the Simulator.

Mine was happening for Xcode 12 and Xcode 11.7. Seems OneDrive was interacting with the files. I paused syncing and the problem went away. Thanks, Microsoft.

still happening in Version 12.0 (12A7209)