How prevent ID from auto-threading text frames?

I've done this with dozens of ID projects: place the text of one TXT file as one story (= threaded text frames) in ID, and then add more text in this way. So if I place 8 TXT files in ID, my ID file has 8 stories. So far, so good.

With the current ID project, however, it's different. In a new (and empty) ID file, I placed 8 TXT files as described above (files A to H), and there were 8 stories. But after editing some text across these 8 stories (changing, adding, deleting some words and sentences) I discovered that suddenly there were only 3 stories in my ID file: while the text of A and B is in a separate story each, the text of C to H has been auto-threaded into one single story.

I need the text of C to H to be in separated stories as well to be able to make changes that affect the text in a specific story only. Luckily, no text has disappeared, so I can restore the 8 stories by unthreading the first frames of D to H.

My question is: What can I do to prevent ID from auto-threading text frames again? And why were the text frames at the beginning of some of the stories threaded in the first place? (Smart Text Reflow is turned off so this had no influence.)

ID 17.4.2
macOS 12.6.8