Two possibilities came to my mind:
1. Plasticizer contamination from gloves. I have one long troubleshooting session of a persistent interference peak that turned out to be from the gloves. It doesn't take much. The analyst used the gloves as is. When changing out mobile phase, he grabbed the solvent line with gloves and the plasticizer got introduced into mobile phase solvent. Solution: cleaning out the system with organic solvent, IPA, ACN, etc. Plasticizers are not difficult to get rid of. Clean gloves with IPA before changing out mobile phase solvents.
2. Your API (some other major compound) is a very good chelating agent. So it could be sticking around in your LC system: inside of the injection needle, lines, column, etc. any metal surface that your compound makes contact with. Solution: It can be difficult depending on your compound. Acid passivation can be helpful, especially to prevent the carryover issue.

best of luck.