Search a specific table based on a value

NewbieQA - Firstly you should add a project column to each sprint table, then append all of them together in Power Query, this will make the DAX much less complex. Here's some guidance on that: https://learn.microsoft.com/en-us/power-query/append-queries

Once that is done, you can create a variable for the selected sprint, 

VAR _project = SELECTEDVALUE( 'Sprint table'[Project] )

and add that into your FILTER conditions:

FILTER(
'Sprint Table',
AND(
'Sprint table'[Start Date] <= __ClosureDate,
__ClosureDate <= 'Sprint Table'[End Date], 'Sprint table'[Project] = _project
)
)

You might need to change the 'Sprint table' for your 'Main table' columns in this filter code - depending on how your relationships are working. 

If this works, please accept as the solution, it helps with visibility for others with the same challenge. 

NewbieQA - Just change the filter argument to this:

FILTER(
'Sprint Table',
'Sprint table'[Start Date] <= __ClosureDate &&
__ClosureDate <= 'Sprint Table'[End Date] && 'Sprint table'[Project] = _project
)

If this works, please accept as the solution, it helps with visibility for others with the same challenge.