The trouble with unframed panelling systems, as opposed to those that are constructed around a structural frame, is that they rely upon the skins to give the wall its strength and stability. If you lose one skin, or the core itself, or even if you just lose the bond between the skin and the core, the wall loses most of its structural properties and could collapse. This will happen at relatively low temperatures, depending upon the nature of the skins, core and adhesives. Therefore, you are likely to get much less fire resistance from this type of construction than ‘traditional’ framed constructions that use the same number and thickness of plasterboard layers. If there are any holes/gaps in the facings, then this is likely to have a dramatic effect on fire resistance, because it will expose the non-fire resisting core directly to the fire.
The other big ‘fire’ issue with this type of construction is cavity barriers (or lack thereof) – if fire does get into the core it will be likely to spread throughout the entire construction unless the appropriate cavity barriers have been installed. This really needs to be checked, I’d suggest.
At the end of the day, safest to assume that the construction will offer little or no fire resistance, & assess the risk accordingly.