MATLAB: Doesn’t diff() recognise the second variable

Question

syms x(t) m1 m2
xr = x;
xl = 0;
T = (m1 * diff(xl)^2) / 2 + (m2 * (diff(xr)^2)) / 2 % why doesn't it recognise that xr is a function of time?

Why is the different from this?

T = (m1 * diff(xl,t)^2) / 2 + (m2 * (diff(xr,t)^2)) / 2 % gives the expected result

whereas if i do

diff(xr) % it gives me the correct result?

What is going on here?

Answer 1

This works correctly in R2020a (Update 3):

syms x(t) 
xr = x
diff(xr) % why doesn't it recognise that xr is a function of time?
T = diff(xr)

producing:

xr(t) =
x(t)
 
ans(t) =
diff(x(t), t)
 
T(t) =
diff(x(t), t)

I cannot reproduce the error.