Detecting interrupt on Atmega8 from zero crossing detector circuit
While it may not be your only problem, your ISR takes several times longer to execute than the 50ms interval between pulses.

Long delays, especially repeated, are almost always a bad idea in an ISR.  Try pulsing a data pin for a few micro (not milli) seconds or better yet toggling its state whenever the interrupt fires, and monitor with the scope rather than an LED.

Given that your signal has a slow rise and sharp fall you might consider a falling edge interrupt.

Also check if the type of interrupt you are using must be explicitly cleared or reset before it can fire again - some MCUs require this and some do not.

Always use extreme care when working with line-connected circuits.