I encounter system-readmore tags in articles created by CB which are definitely not part of the template (I have checked this in html view).
Furthermore I carried out the following effect, which is reproducible and seems to be a bug:
When creating a new record using the backend of contentbuilder, a record in #_content is created by CB representing a new article. The whole content is written to the introtext column including a leading div tag. This is what we expect.
When changing the record in the data storage via the CB backend, the record in #_content is changed accordingly, but now an additional div tag is written to the fulltext column.
As a consequence Joomla creates the system-readmore tag on the fly as it does always when the fulltext column is not empty.
It seems to me that this is a bug in CB.