Flambda 2 interprets the Is_int primitive as being for variants only.
So on the following example:

let str_len x =
  if Obj.is_int x then
    None
  else if Obj.tag x = Obj.string_tag then
    Some (String.length (Obj.obj x : string))
  else None

The String.length primitive sees an input that has gone through Is_int, so assumes it must be a variant, and simplifies to Invalid.
This occurs in practice in the ExtLib library: https://github.com/ygrek/ocaml-extlib/blob/0779f7a881c76f9aca3d5445e2f063ba38a76167/src/std.ml#L165