It's interesting what you say about the cockpit CoM etc. After moving the rear Hover Engines further back the issue went away. However, ad a little more weight - this time approaching 40 tonnes - and the issue is back. I move the Hover Engines back again just two blocks and things are stable once again. I do also notice my HV has a very slight lean to the left, despite being perfectly symmetrical - which is likely the cockpit as you say. Regardless, how the HV is responding to what are still very moderate cargo weights considering its overall real lifting capacity - the Statistics panel shows this as ZERO, I assumed the lifting ability of Hover Engines isn't shown like it would be for an SV / CV using Thrusters for lift - is just plain odd. Though it does perhaps tie in to the physics cycle you show. It's like the rear of the HV sags a little under weight, which is perfectly fine and expected. However, this is then triggering some response as the game attempts to correct this sag, instantly setting HV to level, it sags once more and then begins to bounce, the rear of the HV going above level and perhaps not being "corrected" due to the timing of it, it then falls and bounces higher... The "correction" often catches it, but sometimes it gets totally out of control leading to the flip. I should try to record this, but I'll need to set up OBS again. Scoob.
Thanks for the tip, I've never used it - are there configurations options somewhere for it so I can set up things like quality, where to record to etc.? I'll have a Google.... Edit: Did a Google, it's part of the Game Bar - had issues with that in the past, but I might take another look. I used to use something ages ago called Plays TV, it was great, but it stopped being free IIRC. Scoob.