Thanks Daniel! This is the statement I've been looking for: "Married and zero on the W-4 works under the assumption that the individual with the income is the only earner in the family." I had married/zero for me and single/zero for my spouse for years and always got some kind of refund until about 3 years ago. Then tried married/zero for both of us last year, and to really mess things up HOH for me... (after all I'm certainly the head of my household! ) Fortunately I only allowed that for 3 months before setting it back, but 2020 was the first time we had to borrow to pay our tax bill and even had an underpayment penalty!?! Now single/zero for both from now on. It seems that this should be stated or more clear when turning in an updated W-4.
