Photoshop Generate Image Assets - Scale Size of image outputted not working?

Hi,

I'm using Photoshop 24.5.0 release on Sonoma 14.2.1,

Generate image assets (i'm trying to scale something to 50% size) seems to not be working? i'm trying to export an 800pxwX400pxh image at 50%... so expecting result 400pxwX200pxh

@50% before the masked layer group doesnt work - 

examples i've tried -

'@50% D+_Tesco_Clubcard_Banners_RenegadeNell_734x260.jpg'

'D+_Tesco_Clubcard_Banners_RenegadeNell_734x260.jpg50%' (this used to drop the quality but not any more it seems)

any clues?