Flame Breath Ignite Compounding Not Working

I've been playing flame breath wyvern since league start, and it's worked out well, but one thing I've noticed consistently is ignite doesn't seem to interact properly with its compounding mechanic.

It doesn't matter how long or how little I channel it on one enemy, the ignite does the exact same amount of DoT no matter what, and I'm built fully into ignite magnitude and damage.

I think this has largely gone unnoticed due to the fact that the hit damage off of flame breath is strong by itself so it hasn't been underperforming enough for people to notice. Something definitely isn't working properly here or the explanation of how the mechanic works isn't completely accurate.