Question about Varta LCD Dual Tech Charger

The manufacturer himself writes that 18650 should fit in this charger. https://www.varta-ag.com/de...
This is also listed in the instructions. https://www.varta-ag.com/fileadmin...

The problem lies with the different 18650 variants. The ones with a flat head don't work, that's what I found out. I make do with a small flat magnetic plate that I put on it. But it's really not great that you can't charge all types of 18650 without tricks.

I'd like to send a fan that sees more that the 18650 cell won't charge.