You are right that you probably did something, and it is almost certainly a seating issue rather than damage to the motor. This is the single most common post-repair complaint on this controller.
The actuators sit in moulded pockets in the shell, and how firmly they are held determines how much of their movement reaches your hands. An actuator that is loose in its pocket, or that has its wire trapped underneath or alongside it, absorbs its own vibration instead of transmitting it into the shell. The motor runs exactly as before and draws the same current, which is why the test shows it firing, but what you feel is dramatically reduced. It is very easy to do: you lift the board to get at the battery, the actuator comes with it or shifts slightly, and on reassembly it does not drop fully back into its pocket.
The second possibility is that the wire is pinched between the shell halves, which can both dampen the actuator and, over time, chafe through the insulation. That one is worth finding for reasons beyond the vibration.
So yes, open it again, and specifically check three things: that the left actuator is fully seated in its pocket with no gap, that its wire routes in its channel rather than under or beside the actuator, and that no wire is trapped between the shell halves. Also confirm the actuator's connector is fully home on the board, since a partly seated connector can give reduced drive.
While it is open, since you will be in there anyway, check that the shell closes without tension, because an actuator sitting proud is also a common cause of the gap-and-creak problem people get after a first repair.
The reassuring part: nothing here is damage, and all three causes are free to fix. If after reseating everything the left is still weak, then compare both actuators by swapping them left to right. If the weakness follows the actuator it has genuinely failed, and if it stays on the left the drive circuit is the suspect, which is a much rarer outcome.
