Ophanim Orbs disappear despite being full health?

I noticed this happens most when I send them against long-range targets, if they don't hit anything they just won't come back sometimes, is this a known issue?
sometimes only one of them will be gone but most of the time its both, so I'm forced to recast the spell despite the orbs not actually being dead which is quite problematic for mana management
https://imgur.com/a/ABigEIm