I can give one more data point on this. I was working on an Asteroids years back and I think I did the same thing. I had just put a brand new diode in the thing. When I tested the monitor it worked good but it had some other problem. I don't remember what it was. But I discharged it and fixed the other problem and put it back. But then the diode was blown and I had to order a new one.
Again, it could be explained by other things. But at the time I remember thinking it was because of how I discharged it. Could be right. Could be wrong.
One thing that isn't widely known about these, is that it often isn't actually the diode that is bad. Most of the time the root cause is actually the springs and contacts, which connect the diode to the wires (which then may or may not cause the diode to fail, but often not.)
One of the main failure modes for these cages (in addition to caps) is that the grease dries out that was used inside the boots, air gets inside them, and the springs rust. It's also partly due to a poor design to begin with, because the leads of the diodes were just bent over and clipped, and the springs just press up against them, touching the diode lead at just a single tiny point of contact.
When the springs rust, that single point of contact begins to go resistive, due to the oxidation. Because of the large voltage present, this generates a lot of heat (even though the current is small), as Power = V^2 / R, (i.e., voltage drop across the load squared, divided by resistance). Because V is so big (12-14kV), even the smallest resistance will end up generating heat.
When heat happens, it causes the spring to rust faster, generating more resistance, more heat, more oxidation, etc. The whole thing snowballs, and the boot eventually burns up. This is why you see most of the boots on these HV cages hard and blackened. (The original ones are soft and transparent).
And if you just replace the diode, you often won't fully fix the problem, as the rusty springs will often easily cause a bad connection again pretty quickly (if not immediately).
In many cases when I refurb these cages, the diode is actually still ok, once you clean it up. I soak the springs in vinegar and then tumble them to remove all of the rust, and clean out the boots and cup-shaped contacts inside the boots with acetone.
Then when I reassemble them, I solder the springs to the diode (see below), to remove the weak link. I also dremel out the cups on the end of the wires, so there is no crud inside them, which gives solid contact between the springs, and the cups they sit in.
In some cases if everything is really burned badly, I'll just cut off the cupped contacts, and solder the diode directly to the wires, and then put the boots back over them (or cover them with black RTV silicone, if the boots are too far gone).
Bottom line - It's important to clean and fix the contact points between the diode and springs (especially if rusted and burned) in order to fix these properly, else you can easily see issues return, even with a brand new diode.