ZMP Controller described an apparent interface problem with ZLC that was operating on ERAM resulting in confused beacon code assignments.

Narrative

I had just been relieved from the R-Side position a few minutes before the incident having moved to the A-Side position. The R-Side that had relieved me noticed an Air Carrier flight; eastbound at FL380 that had been in our airspace for at least 20 minutes and showed that he was on the wrong beacon code; indicating a beacon code in the data block. The R-Side pointed this out; and we briefly discussed whether or not I could have missed the fact that he needed a beacon code change on contact. However; we subsequently noticed a Message Waiting indication; and when we pressed the Message Acknowledgment button there were beacon code changes indicated for three separate aircraft; including the one in question; and two other aircraft that were being handed off from ZDV. As this was an anomaly; beacon code changes only occur between facilities; I printed a strip and took it to the OMIC; explaining the situation and assuming it was an automation discrepancy. After his initial investigation; the OMIC informed us that another area had the same thing happen on several aircraft they were working; and that they believed it to be due to an ERAM issue with ZLC; who was running ERAM. All the aircraft in question had flown through ZLC airspace previously. Recommendation; if this is an ERAM bug; then it needs to be corrected ASAP. Since ZLC is continuously running ERAM; should this same thing occur with moderate or greater traffic; it would cause a great distraction and disruption to controllers working aircraft that had their beacon codes changed in the computer.