External documentation is a signal that a tool is not doing its whole job, or was under-designed. We have a ticket tracking website we use at work, and a OneNote page to track who needs to follow up which tickets where and how. If the ticket tracking software was up to scratch, it would handle that follow-up info on its own, without requiring external documentation.
It's always a pain to keep them in sync, too, which is the primary reason it's a problem. If the tool doesn't hold and manage all the data you need it to manage, you'll end up with at least a second list, extra data entry time and effort as well as occasionally out of date data in one or the other location. So you start mistrusting one data store and don't bother with it any more, leading to its eventual death. If one tool is no good without the other, it will not work to keep two tools manually in sync.
Mokalus of Borg
PS - "Manual sync" should always be a red flag phrase.
PPS - And "red flag phrase" should be a tongue-twister.