hey kubizm, happy to hear of your younglings
i've noticed that with a few Acacia species (though not specifically floribunda), the (very new particularly) phyllodes will respond to moisture conditions, and 'tighten up' again to (presumably) conserve or absorb moisture until the next rain (watering)..the opening up again is i assume to maximize rain collection when they 'think' it may be coming, or want it..
in a few species too the juvenile (bi-pinnate) leaves will respond to light, as well as moisture, in opening and closing slightly...like a slower version of the 'sensitive plant' (Mimosa pudica) ..i'm not sure what compounds or mechanisms are responsible for this kind of legunme leaf movement, or if it's known..interesting search for a rainy day..