It doesn't always warn, in my experience.
90 ft of #6 running from a sub-panel with a ???A main and #? wire of ??ft to the main panel is probably enough to cause a pretty substantial voltage drop even with nothing else running let alone normal loads (especially resistive electric aux heat). You're looking at 4-5V drop just in the 90 ft of #6 @ 40A, plus a drop in whatever the run from the subpanel is to the main panel. Plus a drop when adding in the subpanel loads and main panel loads.
Keep in mind that the NEC suggests a max voltage drop in a run of about 4V. You're definitely seeing more than that at 229V/30A. Service size doesn't help if the run is too long from the main panel.
Sounds like you should have a run direct from the main panel, at the very least, IMO. Generally if it's going to be over 100' or close to it you should bump the conductor size by one (in this case, use #4) to limit voltage drop. Then you can use a junction box near the outlet to drop back to #6 if needed to connect to the outlet.
Otherwise it is likely you'll continue to have voltage sag issues, and the car will do the safe thing and lower the current.