How to prevent automatic deletion of photos on iPhone?

There are some way the photos may have become deleted, even if you did not delete them on your Mac:

• Are you subscribing to a Shared iCloud Photos Library? If yes, any of the other subscribers can delete photos any photos from the Shared iCloud Photos Library, even the photos shared by others.
• If you are syncing your devices with iCloud Photos and delete photos from your iPhone or iPad, they will also be deleted from your other devices. When you are using iCloud Photos to keep your devices in sync, you are working with the same library on all your devices, and any photo you delete on one device will be deleted from all others.
• Anyone who is using the same AppleID for iCloud Photos as you can delete photos from your devices.

When we select photos on our Mac to work with them, it can sometimes happen, that we select more photos than we have intended. Selected photos may remain selected, even if they are no longer shown on the display but have been scrolled beyond the borders of the display. It is important to check the number of the photos selected before we delete photos and send them to Recently Deleted. When you open the Info with ⌘I before you hit the ⌫ button, you will see the number of photos and the date range in the Info, and will know, if more photos are included than you wanted to delete.

Here's another option:

I am sometimes startled to find what I thought to be a pretty good picture in Recently Deleted-- but it turns out it's because I had made a duplicate (usually trying a different crop or edit) and deleted it.

So, are you sure that the pictures you see in Recently Deleted aren't copies of pictures still in the Library?

And one more option:

Photos may have opened an old library, that you have not used in a long time. You can see in the Photos > Fererences > General: Path to Library, which Photos Library is currently open. Make sure, you are using your current system library before importing mew photos.