I would never put the bottle in the dishwasher. Because it doesn't really get clean. The second thing is that the seals suffer.

At the end of the day, it's marketing. Good brands are tested to see how long the thermos flask will keep hot/cold if you put it in the dishwasher 10x, 100x, 200x. One brand then says that you only use the bottle 100 times on average anyway and therefore you can put it in the dishwasher because the quality deterioration only occurs after 200 times. The other brand has higher standards and says "not dishwasher safe".
But the fact is, nothing made of metal should go in the dishwasher (just like wood). The material is damaged and the quality deteriorates. This is really independent of what the manufacturer or advertising says.

I was just about to say that. Apart from that, thermos flasks are built in such a way that a vacuum is created between the double walls for insulation. The hot water in combination with the machine's detergent, which is aggressive to materials, damages the metal and makes it permeable to gases, causing the vacuum to deteriorate and deteriorate - and with it the whole thermos flask.

Therefore, no matter what brand, thermos flasks must not be put in the dishwasher.