Unregulated Push pull converter simulation (LTspice) and flux walking
You've got one MOSFET driven at about 60% and the other MOSFET driven about 40% duty cycle - that puts dc into your transformer and walks the flux.

If anyone reads this focusses on the 2nd picture you'll hopefully see what I mean. I've had to invert the colours and make yellow red to make it viewable more legibly: -



If you are driving a centre-tapped transformer like this you have to keep the average on-time for both transistors the same. Now this sounds like a "ban" on PWM control but it isn't; one way you can PWM the transformer is just reduce the on time for both transistor whilst keeping as short a gap between the two pulses as possible. So you charge current in one direction proportional to on-time, then discharge current (via the other transistor) for an equal length of time, then hold-off for some period until ready to start the re-charge process. This keeps the magnetization current from walking the core into saturation.