If you are consistently being impacted by this bug, I encourage you to migrate your instance to the new Fabric editor and rebuild once for all these pages using the Fabric anchors. This should permanently solve the issue for you!
In case someone looks at this ticket and is misled, migrating to the new editor does not solve the problem. The new editor doesn't support dropping anchors at arbitrary places in the document - unlike many other wiki authoring applications, both proprietary and open-source. Instead anchors are currently only available on headers, what Atlassian has called "linkable headers". You can't link to cells in tables and you can't link to arbitrary places in a new editor authored document. For many users (see e.g. CONFCLOUD-69198 and CONFCLOUD-66773 for some entertaining reading) this is a huge problem.
But sticking with the legacy editor is also no good, because the present bug shows up on legacy editor authored pages seemingly at random, and doesn't go away or fix itself. To fix the problem, you have to manually re-edit every single link that goes to an anchor to make it work again, to stop opening the editor and instead go to where the link points. This can be a huge amount of work in a large and complex document. And it's not guaranteed. The next day the bug might come back, after all your manual fixes. I recently had Atlassian support re-enable page creation in the legacy editor (rather than just being able to edit existing legacy editor authored pages in the legacy editor), and after that, a legacy editor page I'd authored with plenty of internal links was hit by this bug. I don't know if re-enabling the legacy editor template is related or a coincidence, but this bug hasn't been solved.
UPDATE: I have a large doc, created with the legacy editor, with many internal links that I managed to revert to an earlier version without losing anything important. So now my internal links and anchors do the right thing. (Other legacy editor created large docs I have are way beyond reverting to get internal links working again instead of opening the editor.)
But if I open the page in the editor, and do nothing except close it again, I'm told there are Unpublished Changes. If I go back into the editor and view changes, I see that the internal links have all been converted to the "resumedraft.action" documented in this bug. So I discard changes and exit the editor.
I have a need to edit this document, but I guess that's just not going to happen. Which is stupid.
@Graham Gill: Atlassian support told me not to update links that point to "resumedraft.action" but to remove and recreate them. This is annoying, can take a long time to execute, but it seems to work. I confirm also we have that bug with the "unpublished changes" (and we do not use this new editor)