Looking for possible cause for poor regulation in self designed transformer
The DC resistance of the IRF540N could give a clue: -



If the gate drive is 5V, the volt drop will be 0.25V at 20A (4 devices in parallel). Given the numbers in your question I think you are using a 12V supply for the power to the primary centre-tap. That's going to be about a 2 or 3% loss of regulation just for the IRF540Ns.

You primary leakage inductance is a staggeringly high 21uH. At 50kHz this is an impedance of 6.6 ohms. This will be a significant problem to getting good regulation as this inductance can be regarded as not contributing to ampere-turns delivered to the secondary.

I expect you may mean 2.1 uH but even at this level it's going to be an issue (bigger than the MOSFETs).

Not mentioned in the question is the ability of the DC power supply to maintain 12V - this could be drooping to maybe 11.5 volts.

Also notable is the output capacitor - it looks like you may have over-stressed it off-load with the 800V DC output - it's a 450V capacitor - do you think it will be performing as well. Nevertheless I'd consider removing it for doing the lamp load tests because the output will be fairly flat due to the square wave drive and output.