Vulpaphyla sometimes loses all accumulated affinity after dying

[(PSN)Ihasapuppy]

While trying to level my Vulpaphyla, I found that after dying and reviving, it will lose all the mastery it accumulated beforehand, reverting it to whatever rank it was before the mission started.

[JeranTek]

I've been trying to figure out why my Vulpaphyla wasn't levelling but this explains it. Thank you.