Another ethnicity question

[Margaret in Burton]

I’ve been looking into my cousins husband tree. He’s a Jones and I haven’t touched that side. Done his mum and his paternal grandmother.
They aren’t sure where his paternal grandfather was born. They thought it might be Wales. Marriage cert he’s in the army so no clues there except he’s in a Devon regiment in 1924.

Their son shows up in my DNA matches and looking at his ethnicity he has only 3% Welsh. Surely if his great grandfather was Welsh then it should be more than 3%. Am I correct?

I’ve asked them to ask him to check his matches to see if there are any clues.