After turning off Connected Services and the mobile apps our i3 has been stable and so far retained its off-peak tariff times.
Looks like the issue may not be the car after all
We have also realised that the off peak tariff times we set are being changed apparently randomly by our i3.
At around 18:00 we reset the tariff times in the car to 1:00-6:00 and connected to the charging cable but then needed to use the car 30 minutes later, before the charging had started...