Expect_test_collector.set being re-run and causing error...

I get this error initialisation error:

Expect_test_collector.set: already set

after dynlinking this code:

let ()= 
  let module M =  (val Major_mode.define_derived_mode ~parent:Major_mode.Fundamental.major_mode
      mode_name
      [%here]
      ~docstring:"Major mode for interacting with my thingy"
      ~mode_line:"MYMODE"
      ())
  in
  let keymap=Major_mode.keymap M.major_mode in
  Keymap.define_key keymap (Key_sequence.create_exn "<f5>")
    (Command (Command.of_value_exn(Value.intern "grabbit")))

As I'm compiling a cma to be dynlinked against the ecaml library and so not re-initialising any ecaml modules, there must be something about the code above that causes
Expect_test_collector.set to be called again.
But why should this module be linked in at all? I have no expect tests.
If it's linking is unavoidable, is there something that can be done to prevent the above call being made?
Thanks.

Start by reproducing the dynlink initialization error with the shown major-mode code and inspect why Expect_test_collector.set is included or called again. Trace the linked modules and initialization path; done means the CMA dynlinks against ecaml without the duplicate initialization error.