I know it's not a bug per se, but for me it acts like one.
Most of the time when an item is created as TLI, IQ assigns it the corresponding grid source field (I'm simplifying).
However, when the TLI is moved down the hierarchy (moved under another TLI), that field/source assignation is cancelled/deleted.
This can have annoying consequences -- it often does here! -- since this doesn't happen in any obvious way and what was there (the field) isn't anymore just because the item was moved.
E.g. A task moved under another item will arbitrarily stop being a task just because it was moved (no rule says that a task isn't allowed to be moved under another item in the same grid...) and might be disappear from some other grid where it should absolutely remain. This task might obviously be missed etc.
(The "auto-assign" field for all items added in this grid" option from the "manage grids" dialog can help alleviate this problem but it creates another problem while solving : now all items are "tasks" (following my previous example), and items need to be manually unchecked so that they aren't all tasks.)
Maybe an option to keep the TLI assignment? Don't know what the best solution is, but certainly, loosing the field assignment by moving it often not a good idea -- and not coherent with the idea that "all items are equal" (in that case TLIs and SLIs aren't equal : SLIs are lower grade... : - ) )