Bent knees rarely mean tight anything. They mean nobody told the legs what to do, so they went where they were left.
What it looks like
The lower leg drops behind the line of the thigh and the toes point at the ceiling instead of away from you. It is the last thing anyone notices upside down and the first thing a photo shows.
Why it happens
Attention. Everything that feels urgent in a handstand is happening at the shoulders, and the legs are the furthest thing from the problem being solved. They are also the first thing forgotten as a hold gets long and the arms start talking.
What it costs you
Mostly the shape and the score — but not only. Bent knees move mass toward your hands and make the whole line harder to read, for you and for the camera. A straight leg is free: once it is a habit it costs nothing to hold.
How to fix it
Point your toes and pull your kneecaps up, every rep, including the ones nobody is filming. Straight-leg squeezes on the floor make it automatic, and saying it out loud before the kick-up works better than noticing it afterwards.
Drills that fix it
Legs together, toes pointed
Every hold, every session.
In every hold, actively squeeze the legs together and point the toes. Put a folded sock between your ankles and do not drop it.
How Nstand measures it
Nstand measures the angle at your knees. Anything under 168°, averaged over the hold, is called bent knees.
It is a camera and some geometry. It does not replace a coach, a gym or an academy, it cannot see your wrists, your history or what you are about to land on, and it is wrong sometimes. Handstands can injure you. What you do on your hands is your decision and your risk: warm up, train where a fall is safe, and stop when something hurts.