1Password autofill vs MacOS text replacement

I started having a strange problem in the past couple of months - when 1Password was autofilling a userid field with my email address the ending part of the email address was being duplicated. This was only happening on MacOS, not on my iPhone or iPad. I finally figured out that I have a keyboard text substitution set up in MacOS settings such that when I enter the first 3 characters of my email address the entire address is substituted. When I turned this off the problem disappeared.

Did something change in how 1Password enters autofill data such that the keyboard text substitution is triggered? Or did something change in MacOS with how substitution works?

Either way it was very confusing, and since I didn't see any other posts about it here I thought this might help someone else who runs into this.