Acts to me like your port relief valve on the boom lift circuit is simply set at too low of PSI, assuming it has one. With a pressure guage it’s a simple adjustment that I occasionally have to make on my excavator to match the requirements of the tool I’m running with that circuit. That being a new under warranty machine I’d definitely let the dealer check/adjust to proper spec. Possibly that being a m62 with longer stick it needs to be set slightly higher than was necessary with the m59 .
You know, 7G, that's a real possibility. The M59 - and presumably also the M62 - do have those overload relief valves you mention. In fact, there are a pair right there on the boom control valve for the boom circuit. One each for each side of the boom cylinder circuit being protected. That would be a very simple fix.
The function of the overload relief is just as you say - to protect the control valve circuit from outside forces. And extending the dipperstick would trigger that relief since it would appear to be an outside force on the boom cylinder circuit.
There are also overload reliefs on the dipperstick, bucket, and something similar on the swing circuits. All those circuits have a pair of overload relief valves. There are other relief valves that protect the pump pressure side of the control circuit, but that isn't the problem here.
The thing is, it is rare to trigger those overload relief valves. They tend to be set higher than the pump pressure psi so you can use full power on the hoe. Checking them is a simple matter. It just takes an adapter, a tee fitting, and a pressure gauge. Any dealer will have it. The relief valves themselves are easy to swap out. Might as well check the load check valve between the ports while you are there. O rings cost two bucks...
On our BH the overload relief valve is set (by Kubota) so that the whole front end and loader bucket will come off the ground before the boom cylinder moves at all. That's typical. In fact, people break backhoes all the time by loading up the front bucket so that the boom will lift more without the tractor rearing up. I've done it myself.....
Uh, I better add an edit. I have done that - put a load of dirt in the bucket to keep it from rearing. But so far have not broken anything. That's probably due more to good luck than good practice. Definitely not something to recommend.
rScotty