How do I use functions defined with Func in the solve function?

Say I have this function defined using Func:

Define f(x)=Func
              Return x²
            EndFunc

I can use this function in most cases perfectly fine:

f(2)            © 4
f(3)+f(4)       © 24
cos(f(sqrt(π))) © -1
expand(f(x+1))  © x²+2⋅x+1

However, when I try to use this function in solve or cSolve (possibly some other functions as well), it doesn't work as expected:

© I expected 'x=-2 or x=2'
solve(f(x)=4,x)   © f(x)=4
© I expected 'x=-2⋅i or x=2⋅i'
cSolve(f(x)=-4,x) © f(x)=-4

It doesn't seem to expand/evaluate the function call and leaves it as f(x).

On the other hand, if I don't use Func, it works perfectly fine:

Define g(x)=x²   © Done
solve(g(x)=4,x)  © x=-2 or x=2
cSolve(g(x)=4,x) © x=-2⋅i or x=2⋅i

nSolve works for both f and g as intended, so this makes me think it's something to do with symbolic computations.

• Is there any documentation about this difference? I've looked over the relevant sections of the reference guide and the guidebook, but have found nothing.
• Is there a way of defining functions using Func (so I can use local variables and things like that) that still work with functions like solve and cSolve?

I'm using a TI-Nspire CX CAS Handheld (the older one, not the CX II).

I contacted customer support and they responded with

There is not a way to use the function defined the way you did using "func" and "return" with the solve command.