i have seen xkb in a couple of search results (1 and 2) regarding mapping keys in ubuntu 18.04. when i take a look at /usr/share/X11/xkb/symbols/us as mentioned in that second link i see line 84 starts a section of us intl with dead keys assignments. line 128 sounds like the key you are referring to:
key AC11* { [dead_acute, dead_diaeresis, apostrophe, quotedbl ] };
*note AC11 is bracketed by <>, but they make it disappear in my response if i leave them in.
i realize this isn’t an answer as to why your keyboard isn’t doing what it should, but maybe the line is incorrect or perhaps it is possible to map a different key to what you need.