When using the Relative Date slicer and selecting Last X Months the first date in the range is filtered out.
Version: 2.85.681.0 64-bit (September 2020)
Steps to recreate
Open a new PowerBI file
Create a date table
Modeling > New Table
Dates = CALENDAR( 43466, 44196 )
Add a measure to the model
Min Date = MIN ( Dates[Date] )
Add a slicer to the report with the date field
Set the slicer to 'Relative Date'
Select Last 1 Months
Add a card to the report
Add the [Min Date] measure to the card.
You will see that the [Min Date] is 1 day later than the date range, 9/1/2020 instead of 8/31/2020.
If you switch to Last 1 Week or Last 1 Year the low end of the date range and the measure match.
I also posted this on the main PowerBI forum.
Here is an example of what I am seeing.
It seems to be more a case of the diplayed filter (the one on the date slicer and in the filter tooltip) not matching the actual filter (the one applied to the model).
Based on further testing is appears that the slicer date range display and the filter tool tip display are using a fomula like this to calculate the starting date.
DATEADD ( TODAY() , - [X months] , MONTH ) + 1
Here is a screen shot from a file calcualted on 9/30/2020:
And I was able to replicate the dates using the formula and comparing it to the last date of the month.
I have uploaded my sample file here for you to look at.
The issue is reported internally : CRI 207973772.
We’ve identified the issue. As the fix involves lots of evaluation, coding and testing, there is no ETA at this time.