After ensuring Maximilian's safety, Franz ordered him to be confined without delay to prevent any mishaps.
He had no choice but to distrust his unreliable brother. There's a saying that fits well, "You won't die if you don't court death."
Don't think that it's truly safe now. The number of people who want Maximilian I dead could join hands and circle around Mexico City several times.
It's fine within the Austrian Camp, where everyone is an ally, and no one seeks his life. But if he went out, there would certainly be more than a few ready to shoot him in the back or throw a bomb.
Of course, preventing him from interfering with the negotiations was also an important factor. This was a valuable bargaining chip in the talks with The Republican Government, which could be fully exploited to cheat.
As long as Maximilian I does not abdicate, he remains the Legal Emperor of Mexico, and any treaty signed with him carries legal weight.
…
Mexico City, inside the Austrian Embassy.