The action menu subpanel keyboard navigation does not follow the ARIA standards. According to  WAIARIA, menus should uses the following keyboard pattern:

https://www.w3.org/WAI/ARIA/apg/patterns/menubar/#keyboardinteraction

The standard does not explicitly cover submenus like subpanels but menu horizontal bars with vertical menus (similar to a typical desktop application top bar). However, the subpanel navigation should be at least consistent with the regular action menu.

This is the summary of the missing keyboard navigation with quotes from the the WAI-ARIA pattern (the parenthesis parts are additions or omissions for better understanding):

Tab, Shift+Tab and Enter

When a menu opens, or when a menubar receives focus, keyboard focus should be placed on the first subpanel item. Because menubar and menu elements are composite widgets as described in the practice for Keyboard Navigation Inside Components, Tab and Shift + Tab do not move focus among the items in the menu. Instead, the keyboard commands described in this section enable users to move focus among the elements in a menubar or menu.

• Move focus into a menubar (refering to TAB or ENTER): (...) focus is set on the first menuitem (first focusable element in the subpanel in this case).
• When focus is on a menuitem in a menu or menubar (subpanel in this case), (the Shift+Tab) move focus out of the menu or menubar, and close all menus and submenus.

Home + End

When the focus is on a subpanel element, move the focus to the first (home) or last (end) subpanel focusable element.

Home: (...) moves focus to the first item in the current menu or menubar (first subpanel link in this case).
End: (...) moves focus to the last item in the current menu or menubar (last subpanel link in this case).

Escape

Move the focus away from the subpanel and return it to the original action menu item. The spec also forces to close the subpanel.

Escape: Close the menu that contains focus and return focus to the element or context, e.g., menu button or parent menuitem, from which the menu was opened.