How did you calculate the dead time?

As far as I can tell you cannot achieve such a long dead time.
I assume you calculated it based on the timer clock frequency divided by the prescaler.

The dead time generator used Tdts, which is not derived from the prescaled tim_psc_ck, it is based on the kernel timer, tim_ker_ck.

The reference manual gives examples based on an 8MHz clock, and the maximum dead time that can be inserted with this method is 31750ns. There is a register to prescale the Tdts, but only with a maximum of 4. I don't think you will be able to see it with a camera. Maybe if you slow down the entire clock tree to some extreme values.