If you were recording an hour-long interview using the Voice Memos app on your iPhone, but when you turned your iPhone back on to stop the recording, it displayed “Downloading…” and you could find only 36 minutes of audio, there are a few important questions that need to be clarified before determining what happened.
Did your iPhone turn off or shut down while the interview was being recorded? If so, was the recording saved before the device shut down, or was the recording still in progress?
You mentioned, “when I turn on my iPhone.” This raises an important question: why was your iPhone turned off in the first place? Was it switched off manually, or did it shut down unexpectedly, perhaps because of a depleted battery?
If the recording was interrupted before the entire interview was captured and saved, the available 36 minutes may be all that was successfully recorded. However, we cannot conclude that the remaining 24 minutes are permanently lost without investigating further.
The “Downloading…” status may indicate that Voice Memos is downloading the available recording from iCloud. It does not, by itself, establish that the missing portion is being recovered or that the recording was interrupted.
Also, if iCloud Voice Memos was enabled, the recording may have been syncing with iCloud. You could check whether the complete recording is available on another Apple device signed in to the same Apple Account.
As currently presented, your question does not provide enough information to determine what happened during or after the recording. We need to understand whether the iPhone shut down, whether the recording was stopped and saved, and whether the 36-minute recording is still downloading or syncing.
Without these details, attributing the missing 24 minutes to a particular cause would be speculation.