Currently there isn't a reliable approach available that would prevent the formatting removal in the pasted cell. It could be done manually by setting the previous format to the cell after the content is pasted, but that would require calling multiple methods of the Range object
for setting different formatting options (bold, textAlign, background, color, etc.). Here's an example which shows this with just two of the methods (background and textAlign):
background = sheet.range(pasteRef).background();
textAlign = sheet.range(pasteRef).textAlign();
Copy a cell's content from a column and paste it in a cell in another column: the original background and text alignment are persisted in the pasted cell.