Relay stays triggered (latched) with 2 sec interval, 50% duty cycle signal
Relays are mechanical switches it could be possible that the switching time is very less for the internal switch to come back to it's original form. If you are using it to control AC you can consider using SCR or a Triac. If you want to continue using the relay you can put an Optocoupler IC such as MOC3021 in between to solve this problem.