Why some of my messages on iphone 11 Pro are getting deleted on its own from my messages

Hello Usha_T_Patel,

If you're missing messages on your iPhone the best steps to take can be found here: Recover deleted text messages on your iPhone or iPad - Apple Support

How to retrieve a deleted message or conversation

1. In Messages, tap Edit on the conversations page. If you previously turned Message Filtering on, the Edit button doesn't appear. Instead, tap Filters on the conversations page.

2. Tap Show Recently Deleted .

3. Choose the conversations with the messages you want to restore, then tap Recover.      

4. Tap Recover Message or Recover [Number] Messages.

Also, you may have auto delete old conversations enabled to optimize your iPhone storage. You can review settings you have enabled by following the guidance listed here: How to check the storage on your iPhone and iPad - Apple Support

Take care.