Hello,
Sharing an observation from a Joomla 5 → 6 migration, in case it's useful for a future release.
Setup: A Mega Menu top-level item has a submenu column with a menu module assigned. That module has Build Mega Menu turned off, so it's meant to render as an ordinary menu module. Same T4 settings on both sites (T4 System 2.6.1; Joomla 5.4.7 vs 6.1.2).
Observed: On Joomla 5 the module's child items rendered as a plain ul.mod-menu__sub (always visible, as intended). On Joomla 6 the same module renders them as ul.dropdown-menu. Inside a Mega Menu column there's no working toggle for that dropdown, so the child list is effectively hidden.
Note: on Joomla 6, if the module's layout is left as Collapsible default menu, the menu renders blank entirely; switching it to Default is what surfaces the dropdown-menu behaviour above.
themes/base/html/mod_menu/default.php always adds deeper dropdown and emits <ul class="dropdown-menu"> for items with children, regardless of the module's own Mega Menu setting, and doesn't consider the standard Joomla mod_menu parameters (e.g. Show Sub-menu Items / showAllChildren). On Joomla 5 this file didn't seem to be reached for these modules, so the behaviour only surfaced on Joomla 6.
I've resolved it on my side with a template override. Flagging it here in case you'd want to address it upstream.
Thanks for your work on T4.