Even if it's a non-problem now it will be a growing one in future and there are a few problems here. Nuclear power makes up 70% of France's electricity, so even if they normally turn off/down some reactors it will become a huge problem if they have to turn many of them off. It's also much easier to plan for a seasonal variance then a weather one, they get much less notification of when rivers will be too hot and when they will cool down again is unpredictable. Finally it undermines the economics of nuclear power, they already take decades to recoup the build cost, if they even can against renewables in future, being able to run even less pushes that further out.

Most importantly it's a warning to other countries considering nuclear, especially equatorial ones where nuclear power is almost non-existent.