You are running into this bug with first appeared in 5.3.
Specifically for you case, when pose assets are created their animation curves no longer get added to the skeleton, meaning that they are not accessible to control rigs.
Prior to 5.3 they would automatically get added to the skeleton and so, in the control rig you could import the curves from the skeleton and drive them just you with morph curves.
The only workaround at the moment is to manually create you pose asset curves on the skeleton but, if like me, you have several hundred of them, it’s not really a valid solution.
Please feel free to add your voice to the above post and hopefully it will get some much needed attention.