How many of you have managers who grill their employees for coming late?

Maybe it's just me being downright lazy but I've always had this idea that working for a tech company would mean flexible timings and no fixed minimum working hours (as long as you meet your deadlines and get the job done).

I've been working for this small AI/CV startup for about 10 months now and my manager/founder's nephew, who is my age(22) keeps grinding me for not coming on time. I'm not really a morning person (and my productivity decreases significantly whenever I wake up early) but I make sure I meet my deadlines.

Initially, I used to avoid all this BS but now it has started to annoy me. Suggestions?