Can't see "Text Insets" lines (rules) on an old document text frame?

I have this old CS5 document opened in 2024 (File > Open).
Select one text frame and looking for the familiar text inset "rules", I could not see any?
Made sure I was in Normal mode (W).
Made sure I had Frame edges showing (Ctrl + H).

Nothing?

I copied and pasted the text frame into a new document.

Nothing.

I replaced the text with Placeholder text, in case.

Nothing.

I created a new text frame and paste the characters into it and after setting the Text inset values; voilà!
The text inset "rules" were finally visible!
Went back to my CS5 document.
Selected some text out of that text frame. Copy. 

Created new text frame, paste.

Changed Text inset value (Ctrl + B) and voilà! Rules finally become visible.

Lesson learned.

It's tough to have these old demo documents around that have pedagogical value but when trying to use them to demonstrate features in classroom, better beware...