This is not a bug, it is the expected behavior (assuming I understand your question). To make sure I understand your question, you want to assign to 'model' a character array like:
model = char('f','n','u');
and you want to be able to autocomplete this selection? If so, then you have misunderstood how the json file works.
The line:
"type": ["char", "choices={'f','n','u'}"],
is designed to show the user the three available options ('f', 'n', or 'u') that the value of the variable 'model' is allowed to take. If you had three different models, one named 'f', one named 'n', and one named 'u', then you would have the correct setup.
To get what you want, you could change it to:
"type": ["char", "choices={'fnu','abc','xyz'}"],
where I made up two other options 'abc' and 'xyz'. Then in your code, parse them out like:
modelselection = [model(1); model(2); model(3)];
or however you want.
If you don't care about autocomplete, then just remove the part with "choices" and parse out the input however you want. However, this is actually a neat trick to getting autocomplete options for multiple parameters all at once. Normally you would need a separate variable for whatever 'f', 'n', and 'u' represent, but this way you get all three at once, albeit in a cryptic manner that nobody else will understand unless they are familiar with the options.
There is another option you might look at to pass in a structure but it can be tricky to figure out (each field of the structure you pass in is treated as a namevalue pair but must have a corresponding addParameter definition in the input parser scheme which is quite rigid but leads to more compact input notation).
