Not much is different about an ODT created in one application or another. However, from this description it could be that the file you were editing might not have been an ODT file. It was probably a text file, because when you created a file in NC, it ended up being a 0-byte file due to this bug:
Then the 0-byte file was treated as a text file (the file format is identified based on the content of the file, not based on extension). Your Collabora Online version is very old (inferring from the Collabora Office 6.0-29 core part version number it was a 4.0 version), this issue is fixed in recent versions, the 0-byte file will be treated as an ODT/ODS/ODP as expected.
The Nextcloud bug is also supposed to be fixed in their development version.
