A user has added and enabled the BitBucket chat notifications integration to our system, and it is visible under the Admin -> Integrations list.
However, calling the /api/v2/integrations endpoint to list the system integrations does not include this integration, it only contains the one other integration which is set up.
I don't have an explanation on why it is implemented like this (see my explanation after this) but I can answer the rest of your question.
No, it is not a bug.
And yes, it is "normal" not to get the BitBucket chat notifications via /api/v2/integrations endpoint.
I just ran a test on my PureCloud environment and I have the same.
The BitBucket chat notifications is not listed via /api/v2/integrations endpoint.
Checking what PureCloud Web client does when showing the Integrations page (in Admin), it seems that it is making use of a non-public API method.
PureCloud Web client makes 2 requests: the /api/v2/integrations one (public API) and another request (non-public/private API).
The second request (private/non-public) seems to be for webhook chat notifications type of integrations.