It's about context, I think. About what you expect from "calm".
"Ficar" can only mean "become" for "charactersitics/states", so, this meaning does apply here. But it's dictionary meaning "to stay" can also apply in many cases if you have a proper context.
With many qualities that are not as dynamic as "calm", the "stay" meaning fades away. It get's somehow pointless or weird to use it.
- She stays beautiful..... well.... why would you say that? It's not a bad sentence itself, but....when do you use it?
- They stay intelligent.... funny sentence
- They stay calm - Now this is a nice sentence to use in almost all cases.
To avoid the confusion, it's rare to see people using "ficar" for "to remain/stay" in these cases where it's not expected. People would mostly use "continuar" for that, so the sentence doesn't get misleading.
In the previous unit there was another sentence with eu peço. I translated it as "I am asking" and my answer was accepted. As ABFoley says, we would be more likely to use this tense in English. However, here, in a construction that is basically the same, "I am asking" was marked wrong. True, the progressive tene is not used in Portuguese, but a good translation is not necessarily literal
It's causing me a problem as well. In reality "I'm asking you to remain calm" would be used or "I need you to remain calm" but that wasn't accepted, another translation could be "Can I ask you to remain calm?". "I ask you to remain calm" sounds really odd to my English ears, I use it because it is required. I'm commenting so that people that are learning English from Portuguese can see the real world translations.
Yes using WE sounds like a common announcement like on the London Underground during bomb scare. But "I" is very direct and abrupt. Possibly a little impolite. I think this is why when using I and ASK together there will be another word like "All" "Only" "Just", "Can" or "Please"
If I spoke to a lady, would I use CALMA or does it remain invariable?
Eu peço que você fique CALMA