That's odd, I don't see that here (I'm also using Fedora 41). Can you start DCP-o-matic in the terminal (just run dcpomatic2) and see if there are any messages printed there?

There is nothing interesting printed:

(dcpomatic2:288109): Gtk-WARNING **: 22:56:10.141: Negative content width -26 (allocation 8, extents 17x17) while allocating gadget (node button, owner GtkButton)

(dcpomatic2:288109): Gtk-WARNING **: 22:56:10.141: Negative content height -2 (allocation 8, extents 5x5) while allocating gadget (node button, owner GtkButton)

(dcpomatic2:288109): Gtk-WARNING **: 22:56:10.141: Negative content width -2 (allocation 0, extents 1x1) while allocating gadget (node header, owner GtkNotebook)

(dcpomatic2:288109): Gtk-WARNING **: 22:56:14.201: Negative content height -9 (allocation 1, extents 5x5) while allocating gadget (node button, owner GtkButton)

(dcpomatic2:288109): Gtk-WARNING **: 22:56:14.258: Negative content width -33 (allocation 1, extents 17x17) while allocating gadget (node button, owner GtkButton)

First I thought that it might have something to do with my insanely old window manager (I use fvwm-1.24r which is 30 years old, don't ask .. :-)
But I can reproduce the problem with a new account on a clean/new machine under Gnome (both wayland and X) and Plasma.
So it doesn't not seem related to my personal setup.

One more datapoint: when started under the LXDE environment, things are OK (proper minus signs)

And under an Xfce environment things are OK too.

DOM 2.17.28 has the same problem BTW