Version 4.1.0 Bug #46885 is back. If one invokes setCellValue(String) on a cell of type FORMULA, the new value will be ignored. One can workaround this issue by explicitly changing the cell type prior to changing its value. XSSFCell.setCellValueImpl() invokes _cell.setT(STCellType.STR) but _cell.isSetF() remains true. Expected behavior: _cell.isSetF() should return false after changing the cell value from a FORMULA to String.
This is likely related to the regression discussed in bug 63845
As far as I could test this should be fixed via the changes for bug #63845 already, maybe you can verify a recent nightly build to verify from your end as well. *** This bug has been marked as a duplicate of bug 63845 ***