If you need everything between today and one month ago then something like this should work...
$currentObject/changedDate >= addMonths([%CurrentDateTime%], -1)
We can use the addMonths function with a negative number to go back to a previous month.
https://docs.mendix.com/refguide/add-date-function-calls/#8-addmonthsutc
How did you create the $MonthFromToday variable? If the above is not working, it seems to me like the creation of that might be wrong.
$MonthFromToday
addMonths([%CurrentDateTime%], -1)
Xpath:
[ChangedDate >= $MonthFromToday and ChangedDate <= '[%CurrentDateTime%]']
That should do it