Call in the Cavalry 2023

It appears that the source data is in text format. Moreover, the source data is in US format, and trying to copy it is done on a computer with UK settings (or vice versa).
Excel is a very greedy tool when it comes to date conversions. It "likes" to convert to date anything that resembles it. I suspect that after copying, the "dates" in text format are the ones that Excel did not convert because the number of days (in US format in the second place) is greater than 12. For example, the "date" in US 07/15/2022 was not able to convert, while the "date" US 07/04/2022 is able to convert to UK format 04/07/2022.

If the source data were written in date format, the copying would probably run without any problems, because numbers are copied. In the case of date-like texts, some may convert to dates and some will remain texts. It seems that in the case of texts, we should first convert the data to dates using, for example, DATE, RIGHT, MID, LEFT formulas, and only then copy to the destination.

But this is just my guess, because I don't have access to the sample file.

Artik