Look at the signal on the output of the logic gate. I don't like the name you have. I suspect that your logic gate has a 6V supply as a result the thresholds do not correspond to 5V.Thank you for the circuit. I tried to replicate it but unable to get the result that you are getting.
View attachment 242492
But the duty cycle is still not fixed as you have(50%)
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Yes the Schmitt Trigger has 6V logic. Which is the correct one? 2V or 4V or 6V?Look at the signal on the output of the logic gate. I don't like the name you have. I suspect that your logic gate has a 6V supply as a result the thresholds do not correspond to 5V.
My bad. I had R3 fed from Vcc, not vc2.opposite way around your results.