A diode like this is probably for the HV supply and a couple
of volts on, what, 10kV is not going to amount to much. Low
Vf is likely acceptable and slightly higher not much of an impact.
Stacked diodes like this are sure meant to not break down,
so "mobetta" there (as long as it doesn't cost somehow else).
You might like to look at a broad line HV rectifier supplier
and see if any of the P/Ns look like likely smears / scrubs /
other marking damage to a likely (parameters-wise) device.
Like maybe it's Y18LA instead, or something like that and the
crack just spalled off a bit of the brand.