Merge of lookup_options not working

Puppet Version: 5.5.14
Puppet Server Version: 5.3.8
OS Name/Version: RHEL 7.x

In our Hiera data, we have made use of lookup_options to control the merge behaviour, so in our catch-all common.yaml there are entries such as:

---

lookup_options:

  profile::autofs::mounts:

    merge: hash 

This works as desired. However we have some exceptions where we want to override this on a per-role basis back to the original "first" behaviour, so as we have a higher priority role level, I added the following in a role/foo.yaml:

---

lookup_options:

  profile::autofs::mounts:

    merge: first 

If I use the "puppet lookup" command against the correct host, this appears to work as desired, using `--explain-options` I get the following result:

  Merged result: {

    "profile::autofs::mounts" => {

      "merge" => "first"

    }

  }

For any other hosts I still get the behaviour defined at the common level:

  Merged result: {

    "profile::autofs::mounts" => {

      "merge" => "hash"

    }

  } 

All good. However this doesn't seem to actually apply to the lookup itself:

Using merge options from "lookup_options" hash

Searching for "profile::autofs::mounts"  Merge strategy hash

  ... 

It only ever picks up the behaviour defined at the common level.

Desired Behavior:

I would expect to see:

Using merge options from "lookup_options" hash

Searching for "profile::autofs::mounts"  ... 

i.e. (no merge strategy displayed here)

Actual Behavior:

One bit of information that might be relevant is that in order for our lookups to work from the CLI, I have to use the --compile option, so basically something like:

puppet lookup --compile --environment <env> --node <node> profile::autofs::mounts --explain(-options)

We have multiple backends/levels configured in Hiera, a mix of YAML/eYAML and using the HTTP backend too.

I noticed there was PUP-7037 which was raised for similar behaviour but was ultimately traced to a typo. I'm confident this isn't down to a typo.

Edit: After a bit more experimenting, I was a bit confused about where "hash" should be printed vs other merge strategies, the 'Using merge options from "lookup_options" hash' is pretty much a fixed string, it's the presence/lack of "Merge strategy ..." that is the identifier.