瑜伽師地論略纂 12.88

12.88

論云「或於一時具足憶念，或於一時成妄念類」者，有憶得不得，故名妄念類。非全妄念，故言類也。

Where the treatise states, "either at one time possessing complete mindfulness, or at one time falling into the category of erroneous mindfulness": having moments of remembering and not remembering is called "falling into the category of erroneous mindfulness." It is not entirely erroneous mindfulness; hence it says "category."