Accoding to the source code for JDBCAppender in version 1.2.7 in the method flushBuffer the 'removes' list is never cleared just gets add called only. So the list will keep expanding. While the buffer list gets removeAll called so it shrinks. Needs to add removes.clear after buffer.removeAll(removes). Otherwise this might be a memory leak. Do you see the same thing?
*** Bug 16998 has been marked as a duplicate of this bug. ***
Fixed in v1.2.8. Thank you for the bug and the fix.