Just to let you know:
I have some issues with my FP2 and tried to solve them with a new battery.
I bought one from vhbw (“For FP2-BAT01”).
But: there is a mechanical mistake in the design of that battery!
The small part on the battery, for easy lifting it out of the phone is on the wrong position (downside). That’s the reason, why it is too tight in the phone and difficult to remove.
Have you contacted VHBW about it? I would not rule out that this is a manufacturing error (wrong assembly) – which might have affected just this battery or just a batch of batteries.