Mortgage pre approvals do not affect your credit score at all.
You can get pre approved for a mortgage or other loan and it will be a soft inquiry and not a hard inquiry.
When you actually apply for the mortgage and get approved then there will be a hard inquiry on the credit score and credit history.
But having a mortgage pre approval does not affect your credit score.
Also pre approval of mortgages can be denied even after pre approval so it's not a guarantee that you will get your mortgage approved just because you have been pre approved.
Yes in rare cases you can be denied a mortgage after pre approval.
You might be pre approved for a mortgage and then when you get approved the bank or mortgage company might find something else in your credit history or something else and then they may decide to deny your mortgage.
When getting a mortgage don't get too excited about the mortgage approval until you actually close on the house or property.
I've had a pre approved mortgage approved and then a few weeks alter got denied for some reason.
The bank didn't give me a very good reason about why they decided to deny the mortgage but I did go with a different mortgage company and bank and got approved and got the house.
So if you do get pre approved for the mortgage and then get denied then try a different bank or mortgage company.
Just make sure you have a good income and good credit score and good credit history before applying for a mortgage.