Recently, CorelDraw always has problems, so I switched to Adobe illustrator (AI).
The vector graphics in pdf format generated by Matlab seem to be unable to be directly split and combined in AI. Use matlab to export images in epsc format instead (because the color image turns gray when exporting eps images directly, just use the epsc suffix instead. See: how to export color eps images with matlab ).
Then import (place) the epsc file generated by matlab in AI. After importing, select the object, right-click, and ungroup (that is, split).