Hey there OldMyque,
We understand you've noticed that when you open an email and return to view your inbox, it vanishes. We'd be happy to provide information about what may be happening.
It sounds like you have the unread filter enabled. Have a look at the information about it here:
Filter emails in Mail on iPhone - Apple Support
Filter emails
1. Tap in the bottom-left corner of a mailbox list.
2. Tap “Filtered by,” then select or turn on the criteria for emails you want to view.
To turn off all filters, tap . To turn off a specific filter, tap “Filtered by,” then deselect it.
Regards.