If there are a large number of future releases, then they will not all show on the dropdowns when trying to split a story.
This is caused when there are more than 200 future releases. The page populates the dropdown with release sorted by start date descending and only requests 200 releases.
Release: OPHUB299000-6.12-Additional OpsHub-Integration
The solution is to reduce the number of open releases, or to apply the release once the story has been split.