It's not the timer it's cheap light or malfunction. Except you have a digital timer with a dim function or something exotic. Common, mechanical timer are on or off, perhaps never on or never off, but nothing between.
I'll spare a halfway right explanation but it's common and discussed - I'm sure you find some discussions about lights that dim sometime after power gets shut off. ("ghosting" "afterglow")
Regarding the "droopiness" if you have autoflowers sometimes I observed some are not going into energy saving mode ever. On the other hand, I have photos which somehow watch out for clouds and immediately drop everything they have ;-)
Is it a problem, for autos no.
For photos, I'd say depends. If you have outdoor plants and are not in the perfect environment you'll have light pollution. Sometimes they take this personally and herm away, others are more stable/get used to slight irritations. Suppose that goes for indoor as well.
Best of luck :-)