With the longer burn before core stage separation, was this a more energetic reentry than they have done before on a 1st stage recovery attempt?
Yes
Have a look at the broadcast, and take special note of the altitude and velocity telemetry of the centre core/upper stage at staging.
It was 9500 km/hr at an altitude of 90km. Normal staging for a Falcon 9 is about 5,500 @ 115km for LEO and 7,500 @ 100 km for a launch to GTO. The centre core was travelling much faster and lower than usual, narrowing the margins for error.
Another thing to consider is that when cores land at sea on the drone ship, they usually use the existing ballistic trajectory (no boostback burn) and simply have a re-entry burn and landing burn. However at this case, with the extra velocity, the ballistic trajectory would have taken the centre core to over 1000miles off the Atlantic Coast, so they decided to use a boostback burn.
Perhaps the faster speed lower altitude and the need for and additional burn led to a miscalculation of the amount of triethylaluminium it needed to ignite the engines three times.