Tools like sheets and excel will infer datatypes and in some cases apply formatting automatically.
You can see that the actual timestamp value is there, it’s just being truncated and formatted as standard date by default.
To escape that formatting it needs to be a string.
A calc column like this in Omni =TEXT(C1, "yyyy-mm-dd hh:mm:ss") will do force excel or sheets to display the full timestamp. Doing this for core reports you know will be exported can help users avoid confusion and extra steps.
It may be possible in the future for Omni to be able to apply formatting to the download but at this time we have not verified it’s possible nor added to any specific time in the roadmap.