From the comments, the post link in the Share menu is that of the poster’s instance.
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I have been using eternity and haven’t noticed any issues.
Would you be able to share the exact steps so that I can replicate it from my side here?
Enable the aforementioned setting.
Find a post made by a user from another instance.
Share from the feed page: link will be to your own instance.
Share from the comments page: link will be to the OP’s instance.