Microsoft Excel (and similar cell editing programs) have built in tools to assist the user in their tasks. One of the many operations available to the user is scientific notation. Shopify ID's can often be mistaken for these operations depending on the ID and how it is set up.
This can become an issue with Custom Fields when importing field values into the app. When you create the CSV in Excel, Google Sheets, etc. and the ID gets reformatted into Scientific Notation, follow the steps below.
In most cases, you can remove the ID column from your import. It is optional and including it does speed up importing.
Alternatively, you can select your data, right-click, and click "Format cells..." and choose Number.