Using a router bit that long is putting more downwards force than normal on the router which is making it want to tip.
One possible solution would be to raise up your anchor points to keep the belts more parallel to the work. Does that seem like an option?
This is because the firmware wasn’t taking into account the extra height from the z-axis being up in the math. This bug should be fixed in the next firmware release (thank you for pointing it out before). That release is still in testing, but if you want to try it it is here: Interstitial Firmware Releases - #585 by bar