The messages of the MicroPython interpreter (stack traces) and print() output appear in the lower section of the IDE. All line breaks are suppressed there. So everything appears in one very long line with a horizontal scrollbar which is hard to read.
However, the output looks fine in a terminal program connected to the virtual COM port.
(made with another Python script)
I noticed the issue with the online IDE on Debian Linux and Chrome browser, and also in Windows 10 with the local installed IDE version 5.3.0.