Can you try adding
'components' => array(
[…]
'twigRenderer' => array(
'sandboxConfig' => array(
filters' => array(
'striptags',
),
),
),
[…]

),

in your config.php file ?

If it don't work : try with function (2 times for one function)

Adding filter in config work, but really seems to be a needed fiter.
https://www.limesurvey.org/forum/design-issues/115436-ls3-x-change-question-style-by-theme-tylesheet-not-inline-via-question-editing#169452

Unsure for function

Fix committed to master branch: http://bugs.limesurvey.org/plugin.php?page=Source/view&id=27303

by the way, it's strip_tags, with an underscore
http://php.net/manual/en/function.strip-tags.php

Release 3.14.2+180807

@DenisChenu, Mazi says:

The applied fix seems to be wrong. We are still facing the issue at latest 5.x and 6.x versions.

This...
https://github.com/LimeSurvey/LimeSurvey/blob/master/application/config/internal.php#L352
https://github.com/LimeSurvey/LimeSurvey/blob/master/application/config/internal.php#L433
...has to be "striptags" which is the official function name: https://twig.symfony.com/doc/3.x/filters/striptags.html

functions is PHP function

https://twig.symfony.com/doc/3.x/filters/striptags.html is a filter, not a function

I think i get the error here

@DeniChenu, I think all that needs to be done is using "striptags" instead of "strip_tags" (with underscore)?!

Fixed : add strip_tags function
Feature : allow striptags filter

{{ strip_tags(aQuestion.text) }} or {{ aQuestion.text | striptags }}

Can you check both when testing ?
https://github.com/LimeSurvey/LimeSurvey/pull/4052

@DenisChenu, both options worked fine at my tests.
Thanks for fixing this so fast!