According to DOC and Miles per Day, starting May 1, Safeway and Albertson’s will no longer allow debit gift cards (Visa/Master gift cards) to purchase money orders. If you still have any Visa gift cards on hand, remember to use them up before then. One thing to note: whether these grocery stores will currently let you buy money orders with Visa gift cards may depend on the store manager or the cashier on duty. If they have already read the memo, they may stop allowing it immediately to avoid the hassle. So it’s best to go use them in-store as soon as possible rather than getting stuck with them.

Based on this note from Miles per Day, it appears the restriction came from the WU side. It also seems they are quite aware of MS activity: not only are debit gift cards banned, but using multiple cards to buy a money order is also prohibited.