This is totally expected behaviour. Check out BSERV-9860, where one of the Bitbucket Server dev team responded to a similar request.
The tl;dr reason is that default reviewers are suggested, not enforced. Even when creating a PR via the UI, default reviewers can be removed before creating the pull request.
If you need to include them on PR's created via the API I'd recommend retrieving the default reviewers via this api so you can then include them when creating the PR.
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