It is absolutely a thing. Forgetfulness, absent-mindedness, short attention span, etc. are common symptoms of ADHD. The meds generally wear off at the end of the day, so by the time the next day rolls around, you're back at baseline, which includes the forgetfulness.

ADHD can be incredibly debilitating and I think it's something that most people don't really grasp despite how well-known the disorder is.