This is what a heater element looks like.
Two things can basically go wrong (electric heaters are extremely simple).
The overload can be damaged and incompletes the circuit.
Second and most commonly, the heater coil burns out (like a lightbulb). Sometimes these wires will come in contact with other parts of the equipment frame. This produces a direct short to ground (as the equipment is a grounded appliance).
The heater that is designed to use two legs of electricity to produce heat now operates on one half voltage capacity.
The heater element is still capable of drawing a specified amount of amperage.
Ohm’s law says if you reduce voltage you end up increasing amperage.
So if you have a 20 amp electric heater that operates under 240 VAC and you eliminate one leg of power, the element can draw 40 A at 120 VAC. (This depends how long the element is after it breaks/burns out).
I tried to find the most simplistic electrical schematic for an electric heater… This one has two heating elements.
Note #1: different equipment is designed differently.
You indicated that there are two circuit breakers in the subpanel.
I will assume (at 99.99% accuracy) that one circuit breaker controls the air-conditioning system and the low-voltage controls. The Second Circuit is dedicated to the electric heaters entirely. Designs may vary.
You will note in this schematic that electrical power comes in on L1 from the circuit breaker, passes through an overload device, passes through the heating element, then passes through a sequencer (that is controlled by the thermostat as Charly pointed out) that completes the circuit back to L2.
If you break the heating element (indicated in blue) and a section of this heating element touches the chassis ground, your heater becomes a 120 V heater.:
It will continue to operate at twice the amperage draw (or less, depending on its length ).