Help Desk Ticket Reference: https://c.na7.visual.force.com/apex/Case_View?id=500A0000007pF4Q&sfdc.override=1
Steps to Reproduce: 1.png: My portlet is on the "Test" page of the portal.
2.png: I click on the maximize button on the right corner of the portlet and the portlet get maximized.
3.png: I click on some other page, e.g. SiteMap.
4.png: I go back to the "Test" page. The portlet is no loger maximized. It looses its state.
project_key: JBEPP

1.png: My portlet is on the "Test" page of the portal.
2.png: I click on the maximize button on the right corner of the portlet and the portlet get maximized.
3.png: I click on some other page, e.g. SiteMap.
4.png: I go back to the "Test" page. The portlet is no loger maximized. It looses its state.

We expect that in 4.png the portlet on the right were still 'Maximized', but it returns to the 'Normal' window state. You can see that the maximize button is there, which suggests that it returns to the 'Normal' window state. The same behaviour happens with gadgets and portlets.

According to JSR-286 - PLT.12.2.2 Portlet Modes and Window State Changes
If the portlet does not set a new portlet or window state at the StateAwareResponse interface the current portlet mode and window state are preserved.

I have already tested the same steps with EPP 4.3, and it preserved the window state. EPP 5.1, on the other hand, doesn't preserve the maximize window state.