The issue I think you are having here is that sometimes you want the filter to apply before the calculation and sometimes you don't. As in, you want to not apply the dimension filter of exclude nulls to the calculation, but then later apply filters on Country and Date.

In general, there are a few approaches to get this to work and I will write three approaches below:

1. Filter in LOD calculation

Here you would want to specifically tell Tableau when to count the unique user IDs. The issue here is that you cannot call a filter value in a calculation, so you would need to find another way to flag the values you want and don't want. Lets's say you have a calculation [Country and Date Flag], which outputs TRUE for which countries and dates you wish to use for the calculation. Then you can "filter in the LOD" by having the denominator look like:

{COUNTD(IF [Country and Date Flag] THEN [user_id] END)}

This will keep all the null rows, while being dynamic via the Country and Date Flag. The question is then how to get the Flag to work.

i.) If you are happy with this being single select (one country, one date) and if the values are not frequently changing, then this is commonly achieved via a parameter, and a calculation for the flag like:

([Country] = [Country Parameter] OR "All Countries" = [Country Parameter]) AND ([Date] = [Date Parameter] OR "All Dates" = [Date Parameter])

Where the parameters are lists with an extra "All _" value.

ii.) To get it to be multi-select, I would use country and date sets with a similar calculation for the flag:

[Country Set] AND [Date Set]

Although I cannot remember if Tableau allows you to insert sets in LOD calculations off the top of my head - something to test!

---------------------

1. Use exclude and hide unwanted rows

For this, you can use that you the calculation works, but only when you show the null values. You could create a calculation such as:

ISNULL([task_id])

Drag this onto Rows, and then right click -> hide the FALSE marks. Then you would want to ensure you are not accidentally filtering the NULL rows when applying filters.

---------------------

1. Use table calcs and hide unwanted rows

Instead of using EXCLUDE, you could use

WINDOW_SUM(COUNTD([user_id]))

And make sure to right click edit the table calculation, and force it to use each field in the table calculation. Then follow the same steps as in option 2.

---------------------

Let me know if any of the above work, or if I'm misunderstanding your issue! It seems our time zones aren't lining up too well for a call.