While the docs don’t explicitly specify a default order, this quote from here pretty strongly suggests that test defaults to first:
This is mostly useful in order to “prepend” a stage to the
teststage that jobs resulting from the matrix expansion will be assigned to.
I’ve always understood that to mean that an order doesn’t need to be explicitly specified unless there is a need to adjust from the default order (where test is first).
in addition, my experience is similar to @richardson-trevor where test has always defaulted to first in the past. i have not personally seen this problem yet, but it is a surprise to me since i do not specify order either and have not had issues like this previously.