Change Settings In Photos

There is not universal auto mode for cropping/straightening. If you click on the Auto button in the Crop mode it will stay on for that particular session of Photos. If you close Photos and reopen the library you'll have to click on the Auto button to get it into the auto mode for cropping.

Tell Apple what missing features you'd like restored or new features added in Photos via Feedback - Photos - Apple

When I open a photo, that needs straightening and open the Crop tool, the straightening will applied, but only, if the photo has not yet been edited in any way. If the photo has already been edited on the iPhone, for example, the automatic straightening will not happen; also, if Photos cannot detect any lines, that should clearly be horizontal or vertical. And I just noticed, that it will not straighten any HEIC images, only the JPEGs.

And the nice perspective correction tools from Photos iOS 13 are still missing completely on the Mac. They have not even been promised to come to Photos for Mac in the Big Sur preview. macOS Big Sur Preview - Features - Apple

As a test launch Photos with the Option key held down and create a new, test library.  Import some photos and test to see if the same problem persists. Does it? (This tells us if the problem is limited to your current library or is more wide spread)

Next, log into another admin user account, Set up users, guests, and groups on Mac,  and see if the problem persists. (This tells us if the problem is limited to your user account or is system wide).

Lastly from your primary user account boot into Safe Mode (How to use safe mode on your Mac - Apple Support) by booting with the Shift key held down and check there to see if the problem persists.  Reboot normally and test again.

NOTE: Safe Mode boot can take up to 10 minutes as it's doing some system cache cleaning, volume verifying and directory repairing. 

It cannot be turned off. As soon as you open the Crop tool the photo will be straightened, if Photos can find the horizon or vertical lines. You cannot avoid it.

My first text the auto didn't work if I turned it off/reset, closed the library, reopened and tested the same photo again. The second time I tested it was automatic. 'Tis a puzzlement.

It is good, that we can revert to the original orientation, after the automatic straighten has been applied, because the automatic straightening can be dreadfully wrong.

léonie wrote:

because the automatic straightening can be dreadfully wrong.

How true!