...then goes to a 20 kHz oscillation between 12 V and 11.7 V.
I think that then Q2 is slowly switching off. The combination of C10, R12 and D14 doesn't give a "hard" voltage to the gate of Q2 is it isn't switched on/off abruptly.
When Q2 is switching off but still in a high resistance state (so it can pull some current from C13) it will help to ...
See if you like this version better.
A set of two integer factor (X & Y), that produce closest integer to a real number (N).
In case, factors of N do not exist, or factors are not usable:
Consider a division, N / X = Y + R, where X and Y are integer, and R is a remainder.
Since, R < X, the smaller divisor (X) is, the smaller remainder (R) results. ...
The 20kHz oscillation hints at the whole thing working, but somehow the 555's output Q isn't making it all the way down to 0V.
I suspect that the problem is whatever you've connected to that output (Q, pin 3), not anything shown here. Or a broken 555 output driver.
I'm certain that it's oscillating, because you see the 4V to 8V excursions at the ...
When power first is applied, C13 is completely discharged, forcing the output high. This is independent of the state of Q2.
You don't give us the Enable voltage in its low state. The 7555 datasheet says its typical threshold value is 0.75 V. If your Enable voltage is higher than this, the 555 will be trying to operate even when Enable is "low" ...