I'm using Moodle 4.2.1 (Build: 20230612) and Questionnaire plugin 4.0.1 (Build - 2023051600) 2022092202 and default Moodle theme Boost.
Maybe your problem is a theme issue, which theme are you using? If not Boost, try to switch to Boost.
Other than that the only help I can provide is for you to export your questionnaire and attach it to you reply so I can upload it to my local Moodle Test site and see if the problem persists.
The slider question type works OK for me, as shown on screenshot below. Please provide more information about your setup; also do you get error if you set Administration Development debug option to maximum?
Also why is your screenshot all greyed out?
Which H5P content(s) are you having this image size problem with?
The issue you refer to was first published 6 years ago and, although it's been labelled as High priority, no-one has been assigned to work on it in the H5P team... I'll let you draw your own conclusions as to the probability that it will be fixed any time soon.
There is no way we can help unless you attach your H5P Multiplechoice Question Set to your message on this forum so we can examine it and try to reproduce the problem!
Have you tried to switch to the default Moodle theme, just in case it's a theme problem?