Professor Onyx (PW) / Extus, Oriq Overlord : inconsistent interaction [Investigating]

Hi,

There is a bugged, yet inconsistent interaction between Extus, Oriq Overlord, and the planewalker Professor Onyx's first ability.

When using Onyx's first ability to fetch and copy a spell card, Extus' magecraft ability triggers, bringing back my 1st creature from graveyard to hand with half mana.

So far so good.

Now Onyx's ability adds 3 mana to the spell copy it produced.

However, those 3 mana are sometimes not added to the spell copy, but to the creature Extus brought back instead.

This can be seen in the following screenshot, taken right after using Onyx's ability :

https://imgur.com/a/5gCBqqB

My last creature (Acquisitions Expert) was just brought back from Extus' ability (as evidenced by the double strike it has gained). It costs 6, and is fully charged (3 from Extus + 3 from Onyx), while the spell copy has no mana.

This does not happen every time though, so it's not easy to track. The inconsistency is really annoying, since I can't be sure whether the mana will go to the spell or creature.

The only other notable card I usually have in play (and also this time) that could also lead to this weird interaction, is the Sedgemoor Witch (it also has a magecraft ability which, although not directly related to mana/hand, triggers before Onyx's +3 mana resolves).

-Mildaene.