The two through hole pads by C19/20 arent soldered and the one above it looks to be a bad joint
jumpers on the right arent connected in the same spot
extra traces by the IC
components can die, especially ICs and capacitors from ESD and drops respectively
PCBs themselves can sometimes just be faulty from the factory and have damaged traces.
Another thing is that small ICs like that tend to be fairly difficult to stick down all the pads. Reworking that might be a good option if checking the above doesnt work
It is an N channel FET. The concept is called “bootstrapping” since Vgs needs to be greater than Vth for the MOSFET to be on. When the FET is on the high side and you want the full 9V on the output, you use the diode to charge the capacitor, and the other side of the cap is 0V. Then, when the other side of the cap is connected to 9V, the charge on the cap can’t go anywhere so the voltage on the other side jumps to 18V. This creates a Vgs of 9V. Ideally you would have something better to drive the gate to fully turn off the FET, but I just used a quick and dirty driver where the bootstrap capacitor directly feeds the gate instead of being the input to the driver. Because if this, the Vgs doesn’t drop completely to 0
My gate driver is fairly crude but you could probably make something a bit better with a PNP transistor and either pull it down or leave it floating, or instead use a szaiklai pair
Signal generators have an internal impedance of 50 ohms and expect a load of 50 ohms when the 50 ohm setting is selected. The 50 ohm impedance prevents reflections of high frequency (rf) signals as it changes mediums. Ideally you should not be using a signal generator to drive an output, but instead buffer it or use an amplifier. If you use high-z the output voltage is exactly what you set it to (e.g. 5Vpp) but with the 50 ohm setting the applies voltage is twice that with an internal resistance of 50 ohms.
I was referring to it more as a difference which might be the source of the issue. For example, one via is fairly close to the pad for C16 which could be shorted (probably isn’t, but still a good idea to check). It could be a wiring issue but could also be a board manufacture issue.