This was reported on PTS, and nothing was changed about it.
The Preview button ([R] by default) does not do anything. You preview things by mousing over. But that means that if you want to mouse off while previewing (like if you want to rotate the weapon to get a different angle,
or if you want to change the state of the mounts oh never mind, they just removed that option), then the game thinks "oh, you moused off, so that obviously means you want to stop the preview, right?"

Please don't fix what ain't broke. It is not basically impossible to preview anything with states, and we need to preplan the rotation if we want to look at a weapon or something. Is it... not intended that we be able to get a good look at something to decide if we want to buy it?