Distinguish preparing from transferring
A phone waiting for Wi-Fi and a server converting a file are not contradictory observations. A download can involve preparation on the source server before bytes are transferred to the mobile device. If server activity continues while the phone waits, identify the queued job and its current stage before assuming the phone ignored a data preference.
Read the mobile setting’s documented consequence
Plex’s mobile settings guide says Wi-Fi Only still permits adding items to the download queue and allows the associated server to prepare them, including transcoding when necessary. The preference therefore should not be described as a command to stop all processing on the server. The same guide gives Downloads a separate quality preference, which can affect how much preparation is needed for an eligible item.
Plan a travel queue deliberately
For example, someone on mobile data can queue several authorised personal videos for later transfer over Wi-Fi. The server owner may see preparation work before the phone begins receiving the files. Coordinate that workload with the owner rather than repeatedly re-queuing the same items. When connectivity is appropriate, inspect completed downloads on the actual phone before leaving. A queued or prepared item is not yet proof that a playable offline copy exists locally. If the behaviour differs, report the client’s Wi-Fi Only state, the job stage and source server context without exposing private access addresses.
Questions, answered
Does Wi-Fi Only stop the source server doing preparation?
Plex says queued items can still be prepared on the server.