I know that I personally can't be in anything that normal people would call "hot water" for all that long without heat stroke kicking in and bam, down I go, but I'm also really susceptible to heat stroke.
It is possible, since Japanese tend to just sit, soak and relax in baths rather than clean (that's what the showers are for,) that they're simply not paying attention and overheat their bodies. High temperature + high humidity = heat stroke super easy.