Hey, I’m not sure we’d be able to prioritize this any time soon as we’d need to make the fix in Discourse code base and then wait for them to port it to the stable branch or make a stable release. The impact of this will be low since invite feature in private DMs is not used frequently here.
If you care strongly about this issue and want to expedite resolution, I recommend reproing this on another Discourse site on a later version, and you could let the Discourse team know about this and/or you can submit a PR to their source code to resolve it.
My hunch is that the ability to delete at all is the issue here and that is what should be changed.