The complication with discussions on this topic is that we really don't know exactly how loud and how high of RPM's are actually being experienced by the person reporting the problem. We all interpret the OP's statement, "when the EV battery runs out and switches to ICE the engine revs very high and loud" based on our own experience with our cars, we assume that the person reporting the problem is hearing the same thing that we do but is simply not used to how hybrids work. While that may be true most of the time, there have been people over the years who have reported abnormally high RPM in their Clarity, something that most of us haven't experienced, at least I haven't. Terms like redlining have been used to describe it, alarming is another word used. We don't hear of this extreme situation very often anymore, one theory is that it gets cleared by 12V reset, which some owners may have done for other reasons.
However for any particular situation being reported it's hard to know what is going on unless we get more specific details from the person reporting the problem. In this case the OP stated, "the engine revs very high and loud", but we really don't know what that means. It could be just the somewhat annoying higher RPM that we all experience at times when EV miles are 0, or it could be one of the more rare redlining situations that people in the past have reported. No way to know unless we hear for ourselves what sound their car is making, but that's not really feasible. Even if they posted an audio recording we probably wouldn't be able to determine much. What would help is if someone who is experiencing the problem used an OBD-II device so they could report the actual RPM's, but probably few people are going to do that.
My guess also is that the OP is experiencing normal behavior, but we really don't know for sure.