What you are probably experiencing is a fault of the method they use to “measure” cadence. They look at the “pulses” of high and low power that is typical in a rider’s pedal stroke. They can pretty reliably capture those variations and estimate cadence from that.
However, that can break down with riders that are especially smooth, or higher cadences with lower forces. These tend to “hide” the power pulses and getting a “halved” cadence is actually common in these cases.
If you really care about cadence, the solution is adding a dedicated cadence sensor.
This is consistent with what I experience. I tend to spin around 100rpm until over FTP. Under lighter loading the cadence gets more inaccurate, when loaded it is typically OK with a fewer fluctuations. If I force a heavy down-stroke the pedals, the cadence the reading will return. As I spent years trying not to that, it is not something I need to re-introduce! Usually, I get cadence from my PowerTap pedals, but they are being serviced, so had recently been using the H3 directly. It doesn’t change the workout (I know what I am doing), but can get rather annoying - especially in the graph and any stats afterward. I have a cadence sensor somewhere, just need to find it.