Number format resets when unrelated changes made to range in Format Cells dialog
Input numbers in two or more contiguous cells. Change the number format from the default on one or more, but not all, of those cells. Select all the cells. Open the Format Changes dialog. Make one or more changes in tabs other than the Number tab. Click OK or Apply. The changes will be applied, but the number format on all the cells in the range will be reset to the default.
No other settings from the dialog are similarly reset. If the number format is consistent across the range, it is not reset. The equivalent toolbar buttons for these settings do not reset the number format, but of course they do not cover the full range of possible settings (especially for borders).