This is not really a bug, but it could be considered as such, because this is debatable.

I would expect the "Select All in All Layers" (Ctrl-AltaA) command to select all the objects, including the ones that are tagged as hidden in the Layers and Objects manager (with the closed eye icon).

This is of course debatable, because one may argue that, if an object is hidden, it's because one doesn't want to select it... Well, to me, the "lock" feature is the right answer to this. So, since there is this feature to lock some objects, then the ones that are not locked should be movable, even if they are hidden.

Such a change would allow to move all the objects of all layers, without having to unhide them.

Another solution would be of course, to have a command to "Select All in All Layers, including hidden layers/objects", but that sounds a bit excessive to me.

There has been debate on this, as well as related operations like snapping. 

One way to select all in all layers including hidden is to use the search tool, which has filters for hidden, locked, object type etc.

I put in a "feature request" to add filters to the objects/layers dialog. 
https://gitlab.com/inkscape/inbox/-/issues/11451
You might want to add your views concerning this

A related post with possible edit>find/replace solutions:
https://inkscape.org/forums/tutorials/tip-unlocking-all-objects-using-findreplace/

Tyler - yes - snapping when it comes to locked layers. If a layer is locked, you shouldn't be able to snap to objects on that layer. 

https://gitlab.com/inkscape/inbox/-/issues/10911

That bug report is labeled as "feature request".