Subreport will grow depending on how much data needs to display. If the subreport is not given enough room, it will overlap other fields or subreports in the same section. In order to avoid leaving too much extra space, move the subreport into its own section.
It is recommended that you put the subreport in its own section so that it can grow without overlapping other fields.
Right-click on the section where the subreport is and select Insert Section Below.