I am experiencing the same issue. My custom title block vanishes on a regular basis, and did so in VW2015 as well. It is definitely not a classes issue. It occurs when I edit the 2D symbol, but occasionally happens when populating or editing the fields. I recently noticed that sometimes when this happens the sheet border symbol is still in the resource browser but that the record format associated with the symbol has vanished. I am not certain if this is always the case.