This is what i try to do.

Prior question was NOT mandatory.
So:
ANSWER1 is answered, ANSWER2 and ANSWER3 both empty.
I test this with "is_empty", and indeed ANSWER1 is filled, ANSWER2 and ANSWER3 are empty.

With the answers I am filling a Question Type Multiple Short Text. Further on I have a follow-up question with an array filter on this Multiple Short Text Question.
Subquestion is:
SQ001 {ANSWER1}
SQ002 {ANSWER2}
SQ003 {ANSWER3}

So far, so good.
I want to use the Multiple Text Question as an array filter, so I fill the answer options as well.
I tried filling it with:
{ANSWER1} {ANSWER2} {ANSWER3}
later with{ANSWER1.NAOK} {ANSWER2.NAOK} {ANSWER3.NAOK}
with {if(!is_empty(ANSWER1.NAOK),ANSWER1.NAOK,'')}, etc
I even used the trim function.

The problem is that LimeSurvey as adding a space to the answer.
This space is causing the NAOK to be NOT EMPTY, and my follow-up question thus shows all answers,
including ANSWER2 and ANSWER3 as a empty line.
(If I manually use the backspace, then the space is deleted, causing the NAOK to be empty.)

The !is_empty is working correctly, since the line is not empty, but holding the undesired space.