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.
Hi have this same issue with attributes. I can see the palette but when I want to change a colour for example, the colour menu does not pop up. Sometimes this is fixed with a restart but not always. Was happening before and after I installed the SP.