Characters with "dead_" in the front of the name are meant to do that - this is so that you can make letters with accents, by pressing it once, then pressing another character that can combine with it, like so: ñ ã ũ (These also have dead versions: ^, °, ˙, ¨, ", and honestly many others). If you don't need/want to make accented letters, you can change the name from "dead_tilde" to "tilde" in the rx-51 file. Or vice-versa if you want to swap other characters.