DAX Calcuate rolling sum - powerbi

Question

I have a problem with calculating measure that sums values for 3 previous periods.
Below I attach sample fact table and dict table to show problem I am facing.
date
customer
segment
value
01.01.2021
1
A
10
02.01.2021
1
A
10
03.01.2021
1
A
10
04.01.2021
1
A
10
01.01.2021
2
B
20
02.01.2021
2
B
30
03.01.2021
2
B
40
dict table:
segment
segment_desc
A
Name of A
B
Name of B
Approach I have taken:
last 3 value =
VAR DATES = DATESINPERIOD(facts[date],LASTDATE(facts[date]), -3,MONTH)
RETURN CALCULATE([sum value], DATES)
It produces correct results as long as there is at least one record for April.
When I use filter on segment_desc = 'B'
It produces result as I attached - so we see result in April equals 20, which is obviously not what I wanted. I would expect it to be 50.

Answer 1

Answer to the main question:
time intelligence functions like DATESINPERIOD require a proper calendar table, because they expect continuous dates, without gaps.
Answer to the follow-up question "why the measure shows value for January?"
It's a bit tricky. First, notice that LASTDATE in this filter context returns blank:
So, your DAX measure then becomes this:
last 3 value =
VAR DATES = DATESINPERIOD(facts[date], BLANK(), -3,MONTH)
RETURN CALCULATE([sum value], DATES)
Blank - 3 month does not make sense, so the way DAX resolves this: it replaces BLANK with the first (min) date in the table. In this case, it's 1/1/2021. Then it goes back 3 months from that date. As a result, the final measure is:
last 3 value =
CALCULATE([sum value], {2020-11-01, 2020-12-01, 2021-01-01 })
Since you have no data prior to 2021-01-01, the final result shows only January values.