Error Category override when submitting a new item

Hello,
I'm working in the v2.2 stable release and encoutered a weird behaviour in the submitting of a new item from the item form from the menu item with category override on.
I'm planning on creating a item form where the main category where the item is submitted is read-only and already setted.
I got two categories :
-CatA
--CatB
Current user can create in CatA.
I got a type MyType.
Current user can create in CatA.
All users can assign multiple categories to an item. It's happen even logged as Super User.
i create a menu item for the item form and choose the type MyType and activate the category ACL override with the selection mode on "All above"
So the weirdness begin: depending on the settings sometimes the submission end with an error, sometimes the main category is changed

MyType Main Category Subtree	MyType Main Category Default	MyType Secondaries Categories Subtree	MyType Secondaries Categories Default	Menu item selected Categories	Menu item default Main category	Result	Result Main Cat	Result Secondaries
CatA	CatA	None	None	CatB	CatB	Redirected to index.php?option=com_flexicontent&task=items.add&typeid=14&jqsldnlqsndlkqnd=1 Error when saving item: main category is not in allowed list (form tempered?)	CatA	CatA
None	None	None	None	CatB	CatB	Save successful: MainCategory of item: CatB But no control anymore on the main cat subtree of the type MyType apart from not giving right to edit it	CatB	CatB
CatA	CatA	None	None	CatB,CatA	CatB	Save success but now got both the parent category	CatB	CatB,CatA
CatA	CatA	None	None	CatB,CatA	CatA	Save success but now got both the parent category	CatA	CatB,CatA
CatA	CatA	None	None	CatB	CatA	Redirected to index.php?option=com_flexicontent&task=items.add&typeid=14&jqsldnlqsndlkqnd=1 Error when saving item: main category is not in allowed list (form tempered?)	CatA	CatA

The category hierarchy doesn't seem to be take into account.
Any workaround, code snippet to apply? does that bug already fixed in the v3 release candidate?

Hello

you will need to use v3 BETA6 before i would consider this, since related code has changed / fixed / improved

Backup and then install:

github.com/FLEXIcontent/flexicontent-cck...3.0.0_Beta6_nightly1