To reproduce:
1. Open attached Unity project "Systemicon.zip"
2. Open "Shooter Mechanic" scene
3. Go into Play mode
4. In the Hierarchy, navigate to Player --> Head --> Player UI Camera(Clone) --> Canvas --> Inventory --> Items In Category --> Viewport --> Content
5. Hold alt to make your cursor appear and click "inv" button to close the inventory.
6. Inspect Left and Right input fields in Content's Rect Transform component
7. Keep opening and closing the inventory

Result: Once the inventory is closed (UI scale.x is set to 0 through animation) , Content's Left and Right input fields' values will change to a random number. Open it again, the Left and Right values will again change to a random number, but with some sort of boundaries. After a random amount of inventory closing, upon opening it again the Left and Right values will not change to a fit in the UI thus it appears as though the items in the inventory have disappeared (see attached gif "UIscale.gif")

Reproduced on: 5.6.4p1, 2017.1.2p2, 2017.2.0p1, 2017.3.0b7