Datediff SECOND with DATETIME type and rounding returns questionable results
Think of DATEDIFF as a truncation and a subtraction.

First the given datetimes are truncated to whatever datepart is chosen. In your case this is SECONDS. So '20200203 06:34:50.100' becomes '20200203 06:34:50' and '20200203 06:34:50.600' becomes '20200203 06:34:50'. Subtract these and the difference is zero - there are zero second-sized time boundaries between the two given inputs.

Note that this is truncation and not rounding, and that the answer depends on the granularity of the datepart chosen. It can be confuing. The difference between '20191231 23:59:59' and '20200101 00:00:00' using YEAR, MONTH, DAY, HOUR, MINUTE and SECOND are all 1.

If you want to report to a precision of one tenth of a second you must calculate to the next most precise datepart (millisecond) and ROUND() the result.