> The manufacturers do mention higher than normal leakage currents,

Perhaps there are two scenarios -

1) a 6.3V cap, new, has 9V put on it. Hasta la vista

2) a 6.3V cap has possibly reformed at 3V. Because of its original
voltage rating, it can cope with a quick reform to 4.5V, but may
sustain some damage, damage that may or may not be significant in
the context of the circuit. You may conclude that "batteries don't last
like they used to", when in fact it's a leaky cap, damaged by fresh
batteries. Or the remote doesn't reach like it used to, because the
reservoir cap can't supply transients for the IR LEDs like it could
before

Fascinating

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