Bag can be close with keybind(ESC) while combat
but can't be close by pressing X button on top right frame while combat

same issue in the burning crusade classic

I know the problem is because HideUIPanel() is protected and cannot be called during combat.

Patch 8.2.0/API changes

• ShowUIPanel() and HideUIPanel() are now protected, they can no longer be called by addons while in combat.

I couldn't find a normal solution because I can't find how HideUIPanel() is triggered. But I found a workaround with it. Add this code to any Lua file and the problem will disappear:

hooksecurefunc("HideUIPanel", function(ui)
    if InCombatLockdown() and type(ui) == 'table' and ui.bagToggle then
        ui:Hide()
    end
end)

It will use the ui:Hide() method to close the bag in battle, which is not affected by the combat lockdown.