Steam client version (build number or date): Sept 26
Distribution (e.g. Ubuntu): Manjaro
Opted into Steam client beta?: Yes
Have you checked for system updates?: Yes
When you have the ready to play filter turned on and you're viewing Hidden Games, you can't turn it off. Normal Collections have an extra button to view the entire Collection, Hidden Games do not.
Also, if you view a normal Collection and hit the extra button to view all in that Collection (not just ready to play titles) it then appears to turn off the ready to play filter across the entire Steam client. Which does then allow you to view Hidden Games normally but it's not expected behaviour for it to turn it off across the whole client. You click it to expand that Collection...
On top of that, the ready to play filter button on the left panel only works in normal Collections and not Hidden Games.
Your system information
When you have the ready to play filter turned on and you're viewing Hidden Games, you can't turn it off. Normal Collections have an extra button to view the entire Collection, Hidden Games do not.
Also, if you view a normal Collection and hit the extra button to view all in that Collection (not just ready to play titles) it then appears to turn off the ready to play filter across the entire Steam client. Which does then allow you to view Hidden Games normally but it's not expected behaviour for it to turn it off across the whole client. You click it to expand that Collection...
On top of that, the ready to play filter button on the left panel only works in normal Collections and not Hidden Games.