Need assistance with an Automation trigger expression

I am trying to set a condition for a bot to fire an action if the Before value is not blank and the After value is blank. The expression I thought would work is:

AND( ISNOTBLANK( [_THISROW_BEFORE].[DETAIL] ), ISBLANK( [_THISROW_AFTER].[DETAIL] ) )

The expression is valid, but apparently will not produce the desired result? (Test pane shows both conditions as empty instead of not empty vs. empty)

I see the expression correct. In the pane both lines ends with “is empty” but it also says that the first one is false.

I think you should troubleshoot how this column value is blanked, whether it is really blanked (for example a Number column set to 0 is not blank), and whether there was really a value before blanking the column.