solve('(L*Y*X*D)+(K*(S-A))+(F*X*T)-(F*(H-X)*T)=N', 'X')
is equivalent to
solve(sym('(L*Y*X*D)+(K*(S-A))+(F*X*T)-(F*(H-X)*T)=N'), sym('X'))
so (L*Y*X*D)+(K*(S-A))+(F*X*T)-(F*(H-X)*T)=N would get evaluated as a symbolic expression inside the symbolic engine. The result of that is going to depend upon whether you have accidentally told the symbolic engine to define some of those names already.
For example,
syms X(V)
would define X as a function, and then the meaning of X as being a function would be inherited when you did the solve() of a string. And that could fail due to difficulties in solving with respect to a function.
