Alright, I set the log level right after terminal_open() and got this:
14:16:50.451 [info] Trying to set "window.fullscreen = true"
14:16:50.451 [debug] Group "window":
14:16:50.451 [debug] * "fullscreen" = "true"
Interestingly enough, if I alt+enter while within the window, it fullscreens fine. Even more interestingly, if I set window.fullscreen = true, while the window doesn't get fullscreened, I can then resize it even though I haven't set window.resizeable to true. I'm on 64-bit arch linux, fvwm, I get the same output on both gcc and clang. If it's relevant, my command line is gcc -std=c11 -LLinux64/ -lBearLibTerminal -o test test.c.