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You could try disconnecting it from Strava and reconnecting? Then record a brief activity and try uploading again. I've only had issues like you describe once and I think it was a Suunto issue with their service in the end. It's normally very reliable.
You can download the activity from Suunto and upload to Strava manually so you don't lose a run but obviously not something you want to be doing for every run!
Yeah...I thought I cracked it because the watch was showing as 2019, but no it still didn’t update the activity to last year. I think I’ll go for a quick trot for the school run and see if that works.