To reproduce the bug:
1\. Place parts U1\.1 and subpart U1\.2\, \.\.\. on schematic Sheet 1\.
2\. Create sheet 2\.
3\. Cut U1\.2 from sheet1 and paste it into sheet 2\.
4\. On sheet 2 a component U2\.2 exists now\. Expected: U1\.2 instead of U2\.2 \. Drag and Drop did not work\, too\.
Workaround:
Rename U2.2 to U1.2.
Firefox
85.0
OS X
10.15
EasyEDA
6.4.7