Could be chuggle. Ie, voltage under load when it’s spent drops too low, the circuit stops functioning, load drops to near-0, voltage rises, it kicks in ever so slightly for a “flicker”, then the cycle repeats ’til the cell’s so spent that it doesn’t come back on again at all.

Neighbor whose yard abuts mine has an idiotic spotlight in his yard, where the sensor must be in the beam or at least catching enough light from it. Turns on, sensor thinks, “Ooh, it’s light out!”, then shuts off 5sec or so later. Then, “Ooh, it’s dark out!” and it comes back on another 5sec or so later. And the cycle repeats.

All.

Night.

Long.

Think of the “flicker” as a very fast and thankfully very transient effect like this.