It looks like the function getParameters() ignores global variables, i.e. parameters, that have null as their default value. See, for example, the Page Forms config - the four variables that default to null are all missing from the "Parameters" listing in the main page infobox. Looking through the code, the relevant line looks fine - the issue seems to be that Lua, when cycling through a table, simply ignores pairs where the value is null (or "nil", in Lua's terms). So maybe the only solution is to replace null with some other value (though I don't know what) either before, or during the load. Maybe there's a different solution, though.
Topic on Module talk:Extension
The set of parameters is now hardcoded in the Page Forms infobox, but you can see the old display here. Note, for example, that $wgPageFormsAutocompleteCacheTimeout, which defaults to null, is not listed there.