The Kunena team has announce the arrival of Kunena 6.2.5 [K 6.2.5] which is now available for download as a native Joomla extension for J! 4.3.x/4.4.x/5.0.x. This version addresses most of the issues that were discovered in K 6.1 / K 6.2 and issues discovered during the last development stages of K 6.2
Usually you get this kinds of issues only if some plugin loads JQuery or other script before Mootools. So I advice you to check if there are new plugins from K2 and if they have settings that fix this issue.