Fault currents on the transmission tie lines of an MMC-HVDC offshore wind power export system are strongly influenced by the converter control at both terminals, exhibiting characteristics such as limited amplitude, controllable phase angle, and blocked zero-sequence current path. These features may cause conventional current differential protection with restraining characteristics to suffer from reduced sensitivity or even maloperation under weak-grid and high-resistance fault conditions. To address this issue, the converter control mechanism and composite sequence fault network are analyzed to theoretically derive the critical operating conditions of the conventional differential criterion. The analysis reveals significant mismatch in the phase-angle difference and magnitude ratio of the terminal currents under weak-grid and high-resistance grounding faults, and further identifies phase-angle mismatch as the dominant factor responsible for the deterioration of the differential operating margin. Based on these findings, an adaptive phase-compensated positive-sequence current differential protection scheme is then proposed. A threshold-based severity index, jointly constructed from and , is employed to identify unfavorable operating conditions. Under such conditions, a bounded dynamic phase compression is applied only to the positive-sequence current with the smaller magnitude, thereby restoring phase alignment between the two terminals. Under favorable operating conditions, the proposed method automatically degenerates into the conventional positive-sequence current differential protection without altering its original operating characteristics. PSCAD/EMTDC simulations verify stable and reliable operation across various fault types, locations, and high-resistance grounding scenarios, significantly improving the sensitivity and reliability of transmission line differential protection in MMC-connected power systems.