You can’t undo years of release if theres problems down the line
Current science says that this release is probably fine, but as you said independent labs and neighboring countries have posed objections based on insufficiency of evidence
"Current science" is really key here because it wasn’t so long ago that science was convinced that heroin could be given to babies, smoking was harmless, and leaded gasoline is safe. Our state of the art has a habit of becoming the next generation’s "how could they be so stupid?"
There have been alternative treatment and disposal options proposed and the Japanese government just happened to chose the cheapest one? That doesn’t pass the sniff test.
Even if the release turns out to be completely safe in retrospect, all of the factors above will cause a significant amount of people to turn their opinions against nuclear power because it sets a precedent for perceived reckless handling of nuclear waste.