We've had multiple Pull Request created in BitBucket that show conflicts in files (and therefore the PR is not mergeable), but there are no visible conflicts - often BB reports a conflict on simple prs where the only difference is a new line, but often the hash in the `composer.lock` file shows as a conflict, when there is clearly only a string change.
Conflicts are indicated by Git, I would advise you to first check if conflicts show locally as well when you run into this issue again. In your local clone of the repo, pull the latest changes for both the source and the destination branch. Then, copy-paste the clone directory to a different location for testing purposes, and try to merge the branches in the copied directory and see if you get any conflicts.
If you don't see any conflicts, I would suggest creating a ticket with the support team to look into this. You can create a ticket via https://support.atlassian.com/contact/#/, in "What can we help you with?" select "Technical issues and bugs" and then Bitbucket Cloud as product. In the ticket description please provide the URL of a PR with this issue; the PR needs to be open and not merged yet so we can investigate the issue while it is occurring.