I am cross-posting my answers.microsoft.com submission in hopes of seeing somebody take this on. I'm getting tired of encountering this issue and the extraneous sharing links it generates.
If I obtain the link to SharePoint content using "People with existing access can use the link" and then paste the full URL into Outlook, then Outlook will convert the text to match the title of the file or folder and the URL in the link will match the link I obtained. Great!
However, if obtain that same URL and then select existing text to insert a link to the URL, Outlook will interface with SharePoint to create an unassigned "People you specify can view" link for the item and change my link to use that URL - which then isn't accessible.
Conversely, if I simply paste the full URL into my email, the conversion into linked text using the filename will properly preserve the URL. The problem only occurs if I try to make existing text into a link.
When I view access to the file in SharePoint, I see that a new link has been created for the file with "People you specify can view" permissions, and nobody is assigned permissions for the link.
If I use 'Undo' (ctrl-z) in Outlook, the link will revert to the correct, full URL and the file type icon will disappear. This is my workaround for now.
Outlook should not automatically create a new shareable link, and it's even worse that the new link prevents access for people who would have otherwise had access. A new link should only be created if Outlook detects that one or more recipients don't have access to the file, and if the sender then chooses to resolve that by creating a new link which either assigns access to all recipients or to 'Everybody with the link'.
Running Outlook v2105 on Windows 10 under a Microsoft 365 Business Standard subscription.