I’ve never heard of this before and I’d hazard a guess that it should not happen because it means you’re sending your money to a different wallet.

This could well be some kind of malicious application on your phone or computer. Programs can access and alter your clipboard (where you copy stuff to) programmatically and it would likely go unnoticed. I would get it checked out by someone who knows what they are doing.

Clear this up:

When you copy the address, the address in your paste changes? or the address on the payment page changes?

If the address in your paste is changing, you have a bitcoin virus - a very common one. It will change any bitcoin address copied into one similar.

Most virus scans won't actually pick this type of virus up, so you'll have a hard time finding it unless you know what you're doing.

If there is something actively changing bitcoin addresses on your computer, you need to run a virus scan using malware bytes. If that doesn't find anything, then it's likely undetectable. You'll have to look into your startup (start menu -> msconfig -> startup) It will say "System Configuration" if you have a newer version of windows, just open the task manager using CTRL + ALT + DEL and the startup menu is in there. Look in there for any suspicious looking programs that begin on startup - if the virus programmer has any brains, the program will not be in there.