A cell has the TIME-format : 00:00:00. After typing 00:11: the cell content changes in 00□:. The change is during typing.
Thank you for your bug report. To avoid the issue, you need to temporarily disable AutoCorrection (https://help.libreoffice.org/Writer/Turning_Off_AutoCorrect) *** This bug has been marked as a duplicate of bug 93233 ***