According to my reading of specification, mask can contain any graphical elements or container elements. But it seems that inkscape only masks using first element.
If you wrap mask content in group in example bellow, Inkscape renders it correctly as expected.

My question: is this expected behavior e.g. for historical reasons or should I file bugreport?

Images (not to scale)
Inkscape

Chrome/Gimp



Tested on 0.92.2 5c3e80d, and fairly recent master

After some testing, it seems your theory isn't entirely correct, see screenshot. It seems to work with more than one object only where they do not intersect. When they intersect, their order in the mask doesn't appear to make a difference, which is strange. I don't understand how Inkscape decides which one to pick.



So, yes, there is a problem, but it still needs some more testing.

Can you play around with it and then make a bug report? (or better, check first if there is one already: https://bugs.launchpad.net/inkscape )

Oh, I tried forums but not launchpad. This problem is known from 0.48 back in 2011 https://bugs.launchpad.net/inkscape/+bug/794472

Grouping mask content is fairly easy workaround. Thanks for input about non-overlaping objects. Given lack of action on given bug I dont think I will investigate/document it further.

I've subscribed to the report. Some time 'soon', that bug tracker is going to be closed, and reports that nobody cares about will probably not be migrated to a new tracker. This will make sure I get a reminder to migrate it, if needed.