I thought at that at first, but the alert definitely said it was running my fixed 18 minute schedule. Here's my history which says that as well. Quite odd.
Another screenshot showing Fixed as disabled, but Flex on at 5am.
I could see Flex starting late as my Lawn ran this morning, so the garden had to wait. However, it shouldn't have run a later afternoon Fixed schedule, and should have run the longer morning Flex instead.