I tried your new Galaxian game and there this problem does not occur. I've tried it with two joysticks, both of which contain "rapidfire as long as the player holds the firebutton down" and whenever I leave the button in "Galaxian DX", the rapidfire stops immediately, like it should be. So there it works.
By the way - there in "Galaxian DX" a rapidfire (autofire) doesn't help much anyway, because a maximum of only one shot of the player can appear on the screen at the same time. But with "Galaga" it often helps, because more shots on the screen are possible there and you can shoot several enemies one after the other in Galaga with rapidfire. :-)
Your new "Galaxian DX" is really good, the next great C64 port of a classical Arcade-shooter. WOW!! You know what. If you don't know, what to port next to the C64, then I have a suggestion. The Atari VCS-2600 version of "Phoenix" is fantastic. A little different from the Arcade original of this game (because the VCS2600 couldn't implement it technically back then, i guess), but the Atari-2600 version of "Phoenix" has it's own charm and plays really great. My number-one game on this system since my childhood. To have this Atari-2600 version 1-to-1 on the C64 too, would be awesome! Nothing needs to be changed, "Phoenix" on the Atari-2600 is awesome as it is and exactly like this it should be ported. Maybe you will find time for that at some point in the future, that would be great!!!