Open the Messages app
Find the conversation containing the deleted text messages
Swipe left on the conversation
Tap Delete
Go to Settings
Tap Messages
Tap Keep Messages
Select 30 Days or 1 Year to reduce stored messages
Go to Settings
Tap General
Tap iPhone Storage
Tap Messages
Delete old message attachments if needed
Check Recently Deleted in Messages if available
Open the conversation and delete any remaining message threads
Restart the iPhone
Update iOS to the latest version
Erase and restore the iPhone from a backup if deleted messages still appear