When I'm trying to select sprint for my issue or to add related issues I always see other projects in the list.
How to hide other projects from Sprints and Related issues?
Screenshots with examples
Unfortunately this is not possible on Cloud - see comment from an Atlassian Team member here.
There is a Jira Cloud feature request for this functionality, so feel free to vote on and watch the issue. It was created in 2014 though so I wouldn't expect this to be delivered any time soon.
The workaround suggested in the initial link is:
Since Sprint names are not unique in JIRA, try using unique Sprint names with a prefix of the Project so that it will be easier to distinguish them.
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