Hello on the manufacturer's website it is specified that the 4K resolution can only be achieved with a colour depth of 8 bits at a frame rate of 30Hz.
https://www.delock.de/produkte...

Means the audio extractor is too slow to support 4K at 60Hz. I think the PS5 notices this and chooses a Full HD resolution at 60 Hz instead of 4K at only 30 Hz, because a low frame rate of 30Hz disturbs the gaming experience more than a lower resolution.

If you still prefer to play at 4K with 30 Hz, you have to see if you can set this manually on your PS5. Unfortunately, I haven't found any information on where and how to do this exactly. If there is such a setting option, it is probably in the picture settings. If you need further help, you can also call Sony's support and ask whether such settings can be changed.

https://www.playstation.com/de-ch...

If you still want to connect full 4K resolution at 60 Hz to your soundbar, you can try:
a. Connect the soundbar directly to your PS5 e.g. via Bluetooth if this is supported
b. Connect the Soundbar to your monitor and set the monitor to forward the audio from the HDMI to your Soundbar.
c. Buy another sound extractor that supports 4K at 60 Hz.