A diode rated for 200V is unlikely to go into reverse breakdown at exactly 200V, unless it is a zener. In fact it is normal for the actual value to be higher to accommodate natural statistical process variation. The results here show that the common 1n400x series may all be the same internally (and the actual breakdown voltage is much higher than the spec):
In production, no one measures the actual breakdown voltage (specified as voltage required for given reverse current), except for Zeners. Instead, the ATE verifies that the current is below the specified level at the guaranteed breakdown voltage. For a given diode, the breakdown voltage is higher than the specified value and can be much higher
I'm aware of all of these things. We are comparing failed/changed diodes against known good of the same part. VR, VF, and capacitance at 0V have all changed, but the device(s) are still largely functional. We believe transients my be an issue, but I was hoping that the observed phenomenon might have been seen by someone else before. I'm just pulling on all of the available threads.