So this is not the default behaviour in Excel. This loss of number precision is caused by a workbook-specific setting called
Set precision as displayed.
The setting is shown here in the
Options -> Advanced -> When calculating this woorkbook section:
Turning this on for a workbook will cause precision to automatically be reduced to what is visible in the workbook.
The setting is disabled\off by default and is described in more detail here. From the docs:
Permanently changes stored values in cells from full precision (15 digits) to whatever format is displayed, including decimal places.
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