Ciphering doesn’t improve resistance to interferance, so there shouldn’t be a correlation between ciphering and DCR.
The only possible explanation is at BSC Border : it is usually difficult (depends on vendor software) to perform a HO between cells using different ciphering algo.
A cell in BSC 1 (with A5/1) might not perform HO to a cell in BSC 2 (with A5/0), therefore, without HO, there will eventually be a drop.