Selecting diodes for HV full bridge rectifier
It's possible that with the secondary open circuit, its natural resonant frequency (due to secondary leakage inductance and parasitic capacitance between windings) is somewhat higher than 100kHz (the switching frequency). When you add the diodes you are also adding capacitance to the secondary and lowering its resonant frequency, possibly to closer coincide with 100kHz. This can mean that you now find it easier to to generate higher output voltages and in fact your AC output voltage may have doubled (take care on this of course because you may destroy the diodes and final output capacitor). What you ought to do is try and determine the secondary AC voltage before and after adding the diodes (which are OK speed wise and reverse recovery wise).

Adding yet another capacitor possibly re-tunes things differently and current drops.