I realize that this is very old but came across this thread looking for another answer. I had issues doing multiple calls in a single BPM like this and someone helped me with the following code between. It causes the partbin deferred records to commit which could be the issue.
Erp.Internal.Lib.DeferredUpdate libDeferredUpdate = new Erp.Internal.Lib.DeferredUpdate(Db);
libDeferredUpdate.UpdPQDemand();
See the original thread https://www.epiusers.help/t/invtransfersvccontract-committransfer-fails-when-transferring-the-same-part-one-after-the-other/45979/29