FINAL UPDATE - I promise, 'cos even I'm getting bored now
The "one point short" bug (on the dial, underneath the "Gold achieved") now occurs ONLY on my iPad mini 2, not my iPhone 6s - even though they are both now running the same version of the App (5.18.2), and the same version of iOS (9.2.1).
On both devices, the dial initially runs up (correctly) to 1,580 tier points, but when refreshed on the iPad Mini2 (by dragging the text down and letting it 'spring back') it changes to 1,579 tier points. This can be reliably reproduced, and is clearly a bug.
The bug now does NOT occur on my iPhone 6s (ever, although it did initially), and it is NOT contingent on whether I am logged in to the BA App on only one device or on both.
Obviously not the end of the world, or the highest priority to fix, but software team might want to investigate in slower time in case the 'refresh' subroutine is doing anything else wrong on a subset of hardware.