I tried to search whether this bug was reported in the past. Maybe it's the same as issue discussed here (plus I found a few more similar discussions), but because it's more than a year old discussion and the bug still exists, I wrote this post.
The problem was discovered by by customer, but original data cannot be shared, so I created very simple test case to illustrate the bug. Unfortunately, it's critical issue, blocking migration to CE (they use MicroStation and ORD). I informed my other customers working with similar data (literally, all civil customers) about this threat, because data are displayed differently.
There is 2D model, referencing another 2D DGN and DWG (3D, but with 2D elements at Z=0 in tested case). In the root model, there is a raster attached (tested with WMS, but raster file attached from disk has the same problem). Because the raster is at background plane, in V8i is correctly displayed behind all elements. But in CE U15, it hides elements from DWG:
I guess the problem itself is clear: Even when the raster has to be always at background, it's not. And because the results are different in CE U15, it's very dangerous (and migration blocking) bug, because some geometry can be hidden without any notification. And there is no chance, when typical civil design consists from tens of DGN and DWG files, to check every file whether everything is correct or not.
Quite accidentally I found a dependency: When references Update sequence dialog is used and root model is changed to be first, hidden elements are displayed. But it cannot be used as a workaround in many situations, because such change would break something else.
I will create also Service Ticket for this issue.
The test case is attached (used WMS is public). Can somebody from Bentley confirm the issue, whether it's defect #1064558 or a new one, and if its fix is planned for U16?