First & Second Congo War would be an obvious point for such things to happen, with detachment of Katanga and Orientale region. Rwanda, Brundi, and Uganda basically did do just that during the Second Congo War, with Mousevini - the darling of the Clinton admin until that point- and Kagame plundering the Eastern provinces for mineral and natural resources, doing their damndest to prolong the conflict.
These actions were -obviously- heavily criticized by the international communities with accompanying reduction of aid to the offending nations. So outright partition of Congo, with Katanga and Orientale carved up by Uganda-Rwanda-Brundi is a pipe dream, imo, I don’t think even Mousevini thought that was possible. Most likely, some rump independent puppet state in the Eastern Congo is the most they could hope for, even then, unlikely.
Angola is the other obvious player in this, with their continuing support of the Kabila regime in DRC. If there influence in the region slips, they might grab Bas-Congo -or Kongo Central since 2015, apparently- which they already have tremendous influence in.