Hi @Duncan_Kerr
This issue is related to the fact that you have reached a 1000000 allowed default open sessions and for some reason they are not getting purged (maxSessions parameter).
There is probably a session leak as a result of a bug or incompatibility of the driver and the server.
Please upgrade to latest compatible drivers and see if after a bounce of the cluster the issue persists.
https://docs.mongodb.com/drivers/ - See needed compatible drivers.
Best regards,
Pavel