Hi @josfl:
The fact that undo recovers the text points to user input as opposed to a bug. This could be as simple as using the Enter/Return on the numeric keypad (which enters a Column Break and forces everything after to the next column) vs the Enter/Return on the main keyboard (which starts a new paragraph).

InDesign has other commands that might make the text disappear from that point downward in the file (i.e., Keep Settings and other manual breaks). Secure in the knowledge that undo will bring your text back, the next time it happens please make note of your exact last keystrokes, and before you choose undo:
- enable Type > Show Hidden Characters and
- navigate to Story Editor view (Edit > Edit in Story Editor) and grab a screen shot of the same area of text
Then close the Story Editor window, undo to recover your text, save and share the screen shot and your last keystrokes with us.
~Barb