This is normal behavior when USD or SYNC I/O's clock reference is set to video ref and NTSC. Similarly, if USD or SYNC I/O is locked to PAL blackburst, they will switch to 25fps when put into generate mode. Theoretically either peripheral could generate 30 fps (or any other frame rate) time code when locked to NTSC or PAL video but the resulting VITC would be invalid, there would be periodic repeating or skipped frames. We also align the LTC top-of-frame sync with video frame boundaries with the incoming clock. Thus, if the SYNC I/O or USD are locked to video reference, they will switch to that frame rate when generating. However, the Pro Tools session will remain at whatever frame rate it was originally set to. If this rate is different, the time code display in the Pro Tools software will be off from that being generated from the SYNC peripheral.