I checked this and probably found the bug, but I'm not sure where it actually is:
1. User click Create new filter group and AJAX is executed to menu callback $items['admin/structure/views/ajax/%/%views_ui_cache']
https://github.com/drupalprojects/views/blob/7.x-3.x/views_ui.module#L135
2. Function views_ui_ajax_form is executed with parameters: $js = TRUE, $key = rearrange-filter, $view = views name
https://github.com/drupalprojects/views/blob/7.x-3.x/includes/admin.inc#...
3. $key is reassigned in line 3015 $key = key($view->stack); and is assigned to zero.
https://github.com/drupalprojects/views/blob/7.x-3.x/includes/admin.inc#...
4. Then comparison $view->form_cache['key'] != $key where values are rearrange-filter and 0 which in PHP7 is FALSE and PHP8 is TRUE. The comparison of course should be between rearrange-filter and rearrange-filter.
https://github.com/drupalprojects/views/blob/7.x-3.x/includes/admin.inc#...