I was on the Complex server doing the Battle Tower challenge, essentially just some consecutive battles. I used my Dusk Mane Necrozma and evolved it to Ultra Necrozma. After the battle ended, it still appeared as Ultra Necrozma in my party instead of reverting to Dusk Mane Necrozma. I tried to send it out, and it was Ultra Necrozma still. When I recalled it, it was just Necrozma, not Dusk Mane nor Ultra. The Solgaleo it was fused with disappeared entirely, not in my PC or anywhere. When creating a ticket with the server they said it was an issue with the mod rather than the server, and I haven't seen anyone else report it here. They said it was an issue with fused pokemon, so this issue may extend to Shadow/Ice Rider Calyrex etc. but I've only seen it with my Dusk Mane.