How to change the lock screen date format on iphone 8 - ios 11.2.6, from short to long?

Ok, Peter.

As I can see, you should check again some of your settings that should be as follow

(even if you find that they are ok, try to change them i.e. if something is "off" and off is the right setting, turn it "on" and then back off):

Settings>Display&Brightness>Text Size... at bottom, put it to the middle position.

Settings>Display&Brightness>Bolt text... disabled

Settings>Display&Brightness>Zoom.... standard

Settings>General>Accessibility>Zoom.... disabled

Settings>General>Accessibility>Bigger text.... disabled

When done, force restart your iP8:

Press and quickly release the Volume Up button. Press and quickly release the Volume Down button. Then, press and hold the Side button until you see the Apple logo. When it appears, release the Side button, wait for your device to restart, then test it.

Please let me know how it goes

Regards

Giulio