System.IO.IOException: The process cannot access the file because it is being used by another process.

GPT claims that this is a race condition, which will cause failure to save (the temp file handle is not yet disposed before deleting it).

https://github.com/Flow-Launcher/Flow.Launcher/blob/d0d41c65fc0fae7366ded6b538d5c0129c6523ba/Flow.Launcher.Infrastructure/Storage/JsonStorage.cs#L215-L224

Start in Flow.Launcher.Infrastructure/Storage/JsonStorage.cs at lines 215-224 and inspect the temporary file save and delete sequence. Reproduce or investigate the IOException during saving, then verify that the temporary file is no longer accessed when deletion occurs and that saves complete reliably without the file-in-use error.

Written by the indexing model from the issue text.