Without seeing what you're doing I'm only guessing, but I suspect you're preallocating an array as a double array and assigning a graphics handle into it using subscripted assignment. When you do this, MATLAB tries to convert the thing you're assigning into the array into the type of the thing into which you're assigning.
So if I have a double value:
x = pi
that I'm trying to assign into an int32 array, that double will be converted into an int32 value.
y = zeros(1, 3, 'int32')
class(y)
y(2) = x % double pi gets converted into int32 3
For backwards compatibility, MATLAB knows how to convert a graphics handle into a double value (which is how graphics handles were stored in the original Handle Graphics system.)
z = zeros(1, 3)
class(z)
h = figure
class(h)
z(2) = h % the Figure object h is converted to double when stored in z
get(z(2), 'Position') % Can use that double handle with get to retrieve properties
h.Position
If you're updating that code to use graphics objects you could preallocate the arrays into which you want to store graphics handles using gobjects instead of zeros.
g = gobjects(1, 3)
g(2) = h
