They fixed the DSD playback issue via Roon. Owning both a pair of LS60 Wireless and a pair of LSX II, I had issued support requests with KEF for both speakers separately in early May. They came back to me early July offering a new firmware for my speakers, which is newer than the official latest release on assets.kef.com. A gave them the MAC addresses of both speakers, the firmware was pushed, and now hi-res DSD up to DSD256 and DXD up to 352.8kHz can be played again without any problems.
The firmware I got for the LS60 Wireless is 3.0.132 (instead of the official 3.0.131) and for the LSX II is 3.0.138 (instead of 3.0.137).
If you own a pair of LS50 Wireless II, I trust they have an equivalent fix in stock for you.
Also see my June 3 post here: Roon server 2.66 compatibility issues with KEF LS50 and LSX (ref#A6H8NQ) - #44 by Akio_Ito