Interesting. I've not had that, but after considerable use, I've found the occasional charging issue, where a battery takes forever to charge. At that point, I buy a new set, shifting the old sets down through my series of torches.
When I started using Ansmanns maybe 20 years ago, I did a deal for the team with a supplier. They had a very helpful info sheet on the batteries. They suggested that they lose power at the approx rate of 1% / day. So I've tended to rely on Duracells or lithiums on longer trips, say a month or more.
I guess that's where a Core battery would be better, top it up as necessary.
My climbing son swears by his Core, as it's always fully charged at the start of a long day - no faffing around changing batteries.