JavaDoc for addItem()/addItems() states that when passing a negative position it will be appended to the end. On eu.davidea:flexible-adapter:5.1.0 you got following annotation
@IntRange(from = 0) which prevents passing negative positions

Tests were completely skipped during development.
Of course this should never happen, but happened. From simple stuff to WorkManager (which didn't even have official tests when this repository was released, but now has!), all help is appreciated.

As the default takes too much space, we can improve this by format it using K per thousands e.g. 300K, 500K etc