The stress of 'flexible' work that's actually just on-call all the time

My boss calls our policy "flexible" because technically I can take time off when I need it. But in practice, if I leave early for a school event, I'm working at 11pm catching up. If I take a Wednesday afternoon off, I'm checking emails through dinner. It's not sustainable but I feel stuck because the policy looks great on paper and saying "this doesn't work" makes me sound ungrateful. Anyone else trapped in this weird "flexibility" that's actually just letting work consume all your time?