It could be, of course, the Propulsion Reduced episode is letting you know your 2011 Volt is experiencing normal aging issues...
If you continue to get Propulsion Reduced episodes under normal driving conditions, try using Mountain Mode as a temporary fix to see if it eliminates the Propulsion Reduced episodes. Switch to MM as you head out for the day, and any time you stop, and then start out again.
In a 2011/2012 Volt, when you switch into MM with sufficient charge still in the battery, the ev range estimate will immediately decrease by ~14 miles, but from that point on, everything will operate as it normally would, and when the revised ev range drops to 0, the car will switch to gas operations.
By increasing the size of the buffer via MM, the amount of power maintained in the battery buffer that is available to be borrowed if needed to maintain performance when the engine is running is increased. This might be enough to prevent Propulsion Reduced episodes.
If the episodes continue, here’s what is happening. To maintain battery life, GM has limited the driver to using only ~65% of the full capacity of the 2011-2015 Volt’s battery. A "fully charged" battery’s state of charge (SOC) is ~85%/87%, and the "switch to gas, fully depleted" SOC is ~20%/22%. Some say under normal driving conditions, the switch is made when the SOC drops to 20%, and then the generator recharges it up back up to a minimum maintained 22%. There’s also a "hard floor" to the SOC level - let’s say around 15% SOC - below which the car’s engineering would not let the battery go.
When the battery is depleted, the motor, in effect, uses the generator output instead of the battery as the electric fuel source. Sometimes the power demand request cannot be met by generator output (e.g., you step hard on the accelerator to pass someone), so the motor "borrows" some extra electricity from the battery... when demand lessens, generator output replaces it...
In the Gen 1 Volts, "borrowing" is done from this ~5% buffer of potentially usable power (~0.8 kWh in the 2011/2012 Volt) below the SOC level being maintained by the generator as you drive and the "hard floor" SOC level.
If the motor attempts to borrow an amount of electricity that would bring the battery SOC too close to the "hard floor" SOC, the computer instead puts the car into Reduced Propulsion Mode that requires less power. The ICE starts revving higher to generate electricity to recharge the battery and increase the SOC back up to the maintained minimum level.
For all Gen 1 Volt models, power demands greater than the remaining battery buffer in the normal fully depleted battery can handle and still maintain performance may occur in certain driving conditions (such as when trying to pass while driving fast up a steep mountain road). "Mountain Mode" was created for when such conditions are anticipated... selecting MM increases the normal "minimum" SOC level (wikipedia says to 45% for the 2012 Volt), increasing the amount of battery power preserved as a buffer for use in these conditions so that your Gen 1 Volt can maintain performance under those conditions.
As the Gen 1 Volt ages, something may impact the "full capacity" of your battery. It may generally degrade, or some cells may have problems, and full capacity decreases. Just as the quantity of power in that 65% usable window of power would shrink as the full capacity shrinks, so would the ~0.8 kWh of power in that 5% buffer between "switch to ICE" and the hard floor SOC. The full output capacity of the gas-engine generator may remain as before, but there’s a smaller amount of power in the battery buffer to use for those "extra power" demand requests.
Thus an aging Gen 1 Volt might have a smaller "borrowable" battery buffer when using the range extender, and therefore might endure "Propulsion Reduced" episodes even when you’re not driving in the mountains, simply because the vehicle is getting older. Unfortunately, the warrantee language for Gen 1 Volts suggests up to a 30% degradation in the warrantee period is likely to be within specs, and that’s certainly enough to produce occasional Propulsion Reduced messages.
As Moderator scottf200 points out above, this could be normal aging. Of course, other types of battery problems could also cause Propulsion Reduced episodes. Your service department can advise.