It sounds a lot like the 7M-G(T)E head gasket issue that afflicted the 3rd gen Supra and 4th gen Cressida(also known as the Mark II in Japan) - but people suspected a combination of graphite head gasket and undertorqued bolts to blame for the issue - the fix if a BHG hasn't happened was to retorque the head bolts.
The Japanese switched over to MLS head gaskets for that reason alone - better sealing but they are still using a fluoropolymer and graphite/moly coating while Fel-Pro on their "problem solver" PermaTorquePlus MLS head gaskets are using a different coating that supposedly holds up to rougher surface finishes and the aftermarket environment better.
The 3rd and 4th gen Prius also have cooled EGR not unlike a EPA2004/2010 diesel engine - the EGR cooler is a smaller scale version of what is used on bigger engines but if the EGR cooler causes BHGs on Toyota hybrids, wouldn't they issue a service campaign with an improved part or modify VVT-i operation to provide more of a pseudo-EGR effect? Older Toyota engines with VVT didn't have EGR systems - retarding the intake valves a tiny bit during the start of the intake stroke provided the effect of EGR.
The Japanese switched over to MLS head gaskets for that reason alone - better sealing but they are still using a fluoropolymer and graphite/moly coating while Fel-Pro on their "problem solver" PermaTorquePlus MLS head gaskets are using a different coating that supposedly holds up to rougher surface finishes and the aftermarket environment better.
The 3rd and 4th gen Prius also have cooled EGR not unlike a EPA2004/2010 diesel engine - the EGR cooler is a smaller scale version of what is used on bigger engines but if the EGR cooler causes BHGs on Toyota hybrids, wouldn't they issue a service campaign with an improved part or modify VVT-i operation to provide more of a pseudo-EGR effect? Older Toyota engines with VVT didn't have EGR systems - retarding the intake valves a tiny bit during the start of the intake stroke provided the effect of EGR.