When I try to set a field of type Item, which uses a custom value drawer, the following Item field can also be set to to the same value. The custom value drawer for Item displays an object field of type ItemAsset instead, which is used to set the Item - the value is determined by and compared with the Item's reference to its asset.
A project where the issue is recreated is attached, along with 4 short videos demonstrating the behaviour. Though it happened in a similar way in my original project, the error occurred in a slightly different way here (described in the project with InfoBoxes). The videos "Original 1-3" show the behaviour in the original project and "Recreated 1" shows the different behaviour in this project.