I see the baseline is less than 200k monthly unique devices and there were no huge drops:
https://analytics.wikimedia.org/dashboards/vital-signs/#projects=igwiki/metrics=MonthlyUniqueDevices

Absent trivial errors, such misclassifications of entire countries have been caused in the past by ISP changes. (I think the biggest case was Australia at some point, it was in Erik Zachte's blog.) Things like moving large IP ranges or slices of traffic from one operator to another. If I remember correctly, some big internet exchanges have been built recently in Central Africa, which may have prompted changes.

I'm not sure at what rate the GeoIP information is updated.

Federico