Also, they grouted in the corners and changes of plane. They said the wedi panel is flexible, so caulking is not needed.
Is it true? Thank you.

First, if Wedi is flexible and grout is not...it sort of leads you to understand what the rest of the competent tiling world understands. That a flexible material should be used where something can be exposed to movement. So yes, it should be caulked instead of grouted.

Secondly...if their regular practices require them to always come back on Day Two to regrout...they really need to learn to grout.

So not only did they do a poor job, but sadly they know they do a poor job. It's become part of their standard practice.

Sad.

But in an imperfect world where mongo and billV are not available to do our tiling, I think a second grouting is pretty normal. It happened in my shower. My contactor's employee also grouted the corner, but I asked him to come back and caulk it. But then I learned that some of the old timers (like Jon bridges) use grout in corners. So maybe it's not the worst thing in the world.

We haven't ever hired tile work done. But dh has done some in our houses and he never had to redo the grout. Now he did do a bit of tile work when he worked construction as a teenager and he is reasonably handy. But he is definitely a DIYer and not a master craftsman. So I would be skeptical of claims that it is typical to need to redo grout.

I have done a fair amount of tiling, and have never had to redo the grout. I can't believe that this is normal for professionals.

I have heard that the only time to grout the change of plane in a shower is when the walls are done the "old fashioned" way with chicken wire and mud, otherwise they should be caulked.

If the grout is done correctly the first time, that's all you need. Two bad groutings may not ever stick and may always be a problem.

I'd make them remove the grout and do it correctly. Yes, that's hard to do. It's easier to rip out everything and start over in a lot of cases. That's an option too.

Thank you for the kind advice. Sorry for the late reply.
They insisted to cover the imperfect grout. Sadly, I was not able to ask more. Anyway, the result is not good. I decided to do re-grout by myself. I do not have any knowledge about grouting. Could you give me some advice? Thank you as always.

You paid a professional for a professional job. Make them redo it. Removing grout is a PIA.