Since some months, when I’m using emacs and I do C-c C-t to get the type of something that is defined in the file I’m currently working on, it always says “no definition found”.
I’m not sure if this coincides with the removal of .merlin files from dune.
I don’t think merlin ever says “no definition found”, are you sure you’re calling merlin?
I’d suggest calling merlin by hand, rather than through a binding, e.g. M-x merlin-type-enclosing RET and see what that says.