Sonarr Ep. Import: Partial File Appears, Then Disappears

Version: 4.0.2.1183
Package Version: 4.0.2.1183-ls230 by [linuxserver. io]:
Mono version (if Sonarr is not running on Windows):
OS: Linux
Debug logs: Excerpt (parts were filtered by Pastebin) Xanteen: Sonarr Logs - Pastebin.com
Description of issue: Sonarr is beginning to copy an episode file from my QBT client, then stops. I can see the file appear in the target series directory, then after a few seconds, it disappears.

IMPORTANT:
This is only happening occasionally. Most episodes copy without issue. (I can’t seem to find a common thread between shows that have this problem.)

This is happening on a QNAP server with Sonarr running inside its own container. The QBT client is also running on the same machine in a separate container. This setup has been running fine for a year with this problem popping up maybe 5-6 times.

I’ve checked permissions on the show and episode library folders and they are correct. I tried setting the problem target folder to 777 and it made no difference.

Thanks for any suggestions!

Excerpts from logs for context:
Warn|ImportApprovedEpisodes|Couldn’t import episode /downloads/…
System.UnauthorizedAccessException: Access to the path '/downloads/
|Debug|DiskProvider| .NET failed to move file likely due to known .NET bug, attempting to move directly. '/downloads/
|Debug|DiskTransferService|Rolling back incomplete file move
|Warn|ImportApprovedEpisodes|Couldn’t import episode /downloads/