Since this is somehow customized, I don't think it is a real bug.
Ajaxmode is experimental and so we cannot guarantee everything stays at it is.
Also this seems to only affect your customized theme.
The issue, btw is solved with a simple js snippet in your customisation.
See here: https://github.com/LimeSurvey/LimeSurvey/blob/master/assets/packages/limesurvey/survey.js#L157

Then: LimeSurvey usage is «don't touch anything»

We add twig system but : if you update a little : you must update totally …

Previous system didn't have know issue : YOU remove it without issue … i'm OK to fix issue if there are some but need a mantis, real report and some lss example …

PS : look at forum for "more next button", "index at other place"
About index : the last one is here : https://www.limesurvey.org/forum/design-issues/115844-move-the-question-index

LimeSurvey usage is not don't touch anything, it is only the ajax mode and thus the specific file ajaxify.js, that is subject to change still.
We are looking for the most specific and most stable solution to get things going and the ajax-mode still is experimental.
I'm really sorry, this is affected. I will make sure to put every issue I find and fix into the bugtracker. I really don't want to fight about this.
I will check to revert the change, but I'm pretty sure the issues with IE11 and Edge will come up again then.

Ajax will be soon disabled.