This is not a bug. The Cool VL Viewer uses a fix that went into LL's viewer Sunshine branch and that didn't yet make its way to the official viewer. This fix prevents issues with the camera when the avatar pelvis goes too low and below the ground. If your mesh needs too large an offset, then it's obviously badly designed (it was not uploaded with an appropriate pelvis offset and needs to be re-uploaded properly): please complain to the mesh creator/uploader.
Please, also note that the Hover offset is done to make for the breakage of the Z offset that was caused by the server-side baking code. It is a *temporary* offset (normally used for adjusting laying/crouching/kneeling/sitting-on-groung animations above the ground), it is not meant for permanent avatar size adjustment (this shall be performed either via wearable shoes + alpha if needed, or via the avatar shape height itself (suitable for full mesh avatars requiring to wear a full body alpha), or via the pelvis offset on upload of the mesh avatar); the Cool VL Viewer will reset the Z offset (i.e. the Hover parameter) to zero at each new login and each time the shape is changed (when it is mod-ok), but you can also adjust this offset in near-real time via the spinner in the Movement Controls floater ("View" -> "Movement controls" to show it).
Finally, this offset is also used by the @adjustheight command of RestrainedLove: if you have scripted items that make use of this command and have RestrainedLove enabled, the Z offset/Hover parameter will change under the control of these items.