looking very nice your project!
I’m not a hero in widget programming. But maybe i have some words to point you in the right direction to fix this issue.
is the canvas size of the chat setting up correctly?
maybe it puts the inventory window in position above the screen. where you logically not see it - also if its recovered. because i think you recover the inventory window to it last location.
Maybe you can handle some overlapping events? do you have a mainwidget which holds all further widgets?
else,
could’nt you check with IsInViewport if it is in Viewport when it should - if it isn’t as it should remove it from parent and add the widget again to the viewport. Print out the location of the dissapearing widget to check where it goes 
Keep up the great work!
Greetz from switzerland