It is a familiar situation: a distribution updates Gtk+ to a supposedly-compatible version and applications, here Gnumeric, break.
This time I am guessing that it is incompatible changes to widget theming that renders Gnumeric impossible to use.
I would estimate that this has happened 15-20 times within the GTK+ 3.x series. One or two of those are probably Gnumeric accidentally relying on a GTK+ bug that got fixed, but the vast majority of cases is simply that perfectly fine, existing code stops working.
Imagine the C library changing the behaviour of a handful of functions every release. I suspect GTK+ maintainers would be somewhat upset over that. Nevertheless, that is what is presented to GTK+ application writers.
The question of whether GTK+ applications can be written remains open with a somewhat negative outlook.