Xperia 10 IV headphone jack - puzzling issue!

[mr_cushtie]

Wired headphones have been working fine on my new Xperia 10 IV. Sounds great. Bought some new Flare earphones that can be connected either via 3.5mm jack or via Bluetooth. The latter is fine, but the phone will not detect the jack. 

I've plugged the jack into other devices and it's working fine. I've plugged other wired headphones into the phone, and it's working fine. There must be something specific about this phone and these earphones. 

I've managed to trick the phone into playing the earphones via wired connection by using a jack adapter (3.5mm male to 3.5mm female - there's barely a reason for such a thing to exist, but it does, and I've got one). If I plug the adapter in to the phone, nothing happens. If I plug my old Soundmagic earphones into the adapter, the phone accepts the connection. If I then unplug the Soundmagics from the adapter and plug in the new Flare earphones, the phone will recognise and accept them. They sound great. 

But it will not accept them directly, or via the adapter unless I've tricked it with Soundmagics first. 

Anyone have any idea what the hell is going on here?

Thanks!

Ben