ID 343654 - A shared mask layer loses data and becomes uneditable after unsharing

Problem summary
A shared mask layer loses data and becomes uneditable after unsharing from a group

Customer reported version
3.3v1
Customer reported platform
rhel7

Steps to reproduce

1) Open the attached archive. Within the archive, there are three channels. The 'Mask1' channel contains a shared mask from the 'Color' channel. That same shared mask layer was then grouped and then shared to the 'Mask2' channel. 

2) In the 'Mask1' channel,select the Group layer and unshare

3) Go to the Mask2 channel and select the Shared Color_paint(Mask) layer

4) Undo all the way back either by using [Ctrl]+[Z] or by stepping back within the History palette

Workaround
Unknown. 

Reproduced by support
Mari 4.0v4 - Windows 7, CentOS7
Mari 4.0v1
Mari 3.3v1
Mari 3.2v1
Mari 3.0v1 - Windows 7, CentOS7

Earliest version tested
Mari 2.6v5 - Works as expected

Expected behaviour
The project should return to its original state with layers grouped.

Actual behaviour
The masks seem to lose their paint data and become uneditable