When Aegislash uses the Z Move based on the attack, he attacks with shield form instead of changing to sword form.

Video: https://youtu.be/LjKdpnYM6gM

Hey there!

The correct mechanic is that whenever Aegislash uses any offensive move, regardless of being a Z-Move, it should always change to Blade form.

To replicate the issue you reported, I did the following:
1. Obtain an Aegislash holding a Ghostium Z and the moves Shadow Claw, King's Shield.
2. Set up an NPC Trainer with a Max IV, 0 EV, Neutral Nature Guzzlord
3. Commence battle, and on Turn 1, When Aegislash is in Shield form, use Z-Shadow Claw. Write down the average damage which was dealt.
4. End Battle
5. Commence the Battle again, but this time go into Blade form by attacking with plain Shadow Claw, and then using Z-Shadow Claw. Calculate the average damage dealt from the Z-Move hit in Blade form.

Aegislash should deal between (28.4 - 33.5%) using Z-Shadow Claw, in Blade form however, it only ended up dealing an average of 12.5% whenever the Z-Move would be used in Shield form. The expected Shield-Form damage is (12.2 - 14.4%), which lines up with the results.

This is indeed a bug, and has been forwarded to Internal. Thank you for making this report =)