There is a setting under Settings -> Messages -> Audio Messages section. There is an "Expire" option, where you can select 'after 2 minutes' or 'never'. That is why it disappeared from your message thread. ... Each device that has the message handles deletions independently of the others.

Do voice memos automatically delete?

The Voice Memos settings that you need to set up the automatic removal are in your device's Settings app. ... 1) In Settings, select Voice Memos. 2) Scroll down to Voice Memos Settings and tap Clear Deleted. 3) Choose when to remove deleted recordings.

Do voice memos disappear?

Voice messages will expire two minutes after the recipient plays them, but this time limit can be removed in your iPhone's Settings app. Your voice messages can be as long as you want — there's no limit.