Topological Josephson junctions designed on the surface of a 3D-topological insulator (TI) harbor Majorana bound states (MBS's) among a continuum of conventional Andreev bound states. The distinct feature of these MBS's lies in the 4π-periodicity of their energy-phase relation that yields a fractional ac Josephson effect and a suppression of odd Shapiro steps under rf irradiation. Yet, recent experiments showed that a few, or only the first, odd Shapiro steps are missing, casting doubts on the interpretation. Here, we show that Josephson junctions tailored on the large bandgap 3D TI Bi2Se3 exhibit a fractional ac Josephson effect acting on the first Shapiro step only. With a modified resistively shunted junction model, we demonstrate that the resilience of higher order odd Shapiro steps can be accounted for by thermal poisoning driven by Joule overheating. Furthermore, we uncover a residual supercurrent at the nodes between Shapiro lobes, which provides a direct and novel signature of the current carried by the MBS's. Our findings showcase the crucial role of thermal effects in topological Josephson junctions and lend support to the Majorana origin of the partial suppression of odd Shapiro steps.Topological superconductivity engineered by coupling superconducting electrodes to topological states of matter has attracted considerable attention due to the prospect of manipulating Majorana states for topological quantum computing [1][2][3].Intense experimental efforts have focused on spectroscopic signatures of Majorana bound states (MBS's) in various superconductivity-proximitized systems, including semiconducting nanowires [4][5][6][7][8], atomic chains [9,10] or islands [11] of magnetic atoms, and vortices at the surface of 3D TI's [12].Another key approach to substantiate the very existence of MBS relies on the fractional ac Josephson effect [13][14][15] that develops in topological Josephson junction [3,15]. Theory predicts that MBS's shall emerge in such junctions as a peculiar, spinless Andreev bound state (ABS). Contrary to the conventional ABS's whose energy level varies 2π-periodically with the phase difference φ between the junction electrodes, the MBS is 4πperiodic and crosses zero-energy for a phase π (see Fig. 1a), yielding a fractional ac Josephson effect at frequency f J /2 = eV /h (e is the electron charge, V the voltage drop across the junction and h the Planck constant), that is, half the Josephson frequency f J [3,15].Yet, revealing such a 4π-periodic contribution has proven challenging in dc transport experiments due to the presence of often prevailing, conventional ABS's [16][17][18]. Moreover, poisoning processes -stochastic paritychanges of the quasiparticle occupation number -may obscure the MBS contribution by limiting its lifetime [19,20]. Measurement schemes probing at timescales shorter than this lifetime are thus essential. The Shapiro effect comes forth with the combined advantages of a radiofrequency (f rf ) excitation of the phase that can be faster than the poisoning dynamics [15,21...