Can confirm - preview works normally, then goes away if mouse goes below the top of the second quickbar row/top of the tool/vehicle window. This does not happen on the right of the quickbar.
If the Tool Window next to Quickbar option is on, the same thing happens in the lower left corner.
If more than 2 quickbar rows are enabled, it DOES NOT happen if you're level with the 3rd+row. It MUST be below with the top of the second quickbar row/top of the tool/vehicle window. If only 1 quickbar row is enabled, it still happens at the top of where the second quickbar would be.