Rectangular pattern constraint is lost when applying a fillet to the sketch object
In some CAD packages, this is exactly what I would expect.  Models are constructed by applying operations in a specific order.  The fillet is a separate operation, and is not part of the base rectangle.

The base rectangle is defined.
You pattern the base rectangle.
You define a fillet using the base rectangle as the reference.  The fillet affects only that one rectangle.

OTOH, when you modify the base rectangle all are changed because the base rectangle is defined before the patterning operation.

If you can apply the fillet before the patterning, it may work as you wish.