Using 6.1.0 on Win 10, renum silently fails (does not update/resave the file) if the nfo contains an action B.
I discovered this with a custom action B, and verified it in multiple different nfo files with the example code from the wiki, which is:
2 * 6 07 85 01 01 37 01
3 * 17 0B 03 7F 02 "newtrains on" 00
If I simply change the "0b" to "0c", renum processes the file successfully. If it is then changed back to 0b and the grf compiled, the action B works as expected.