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What version of ODK Collect are you using?
Did you use exactly the same form you have attached to send data to Google Sheet?
I've never heard of such problems. I tried to reproduce it but to no avail. I don't think it's a bug in ODK Collect I would rather say it might be related to formatting in Google Sheets. Have you changed maybe something in columns formatting? Could you maybe try sending data to a completly new spreadsheet?