Top left: decomposition of the different flaring episodes of the B0218+357 major flare in 2012. The original Fermi/LAT light curve is shown in blue, the reconstructed light curve of the leading image μ_γF(t) is shown in red, and the total reconstructed light curve F^tot(t) is shown with a green line. Top right: derived magnification factor ratio for each of the flaring episodes. Black dashed line indicates the approximate value of the magnification factor ratio in radio band μ_rad ≈ 4. The red line depicts the variation of the magnification factor ratio during one of the caustic-crossing events in our simulations, which can be tentatively associated with the observed behaviour. It was computed for R_source ≈ 10¹⁴ cm. This event is shown in lower panels. Bottom: microlensing caustic-magnification patterns that can be associated with the detected variability of magnification factor during the 2012 and 2014 flares. The colour bars represent the microlensing magnification (or demagnification if the value is <1). Distances are given in the source plane. The patterns are shown separately for the leading (left) and delayed images (right). Solid white lines indicate the putative trajectories of the images during the 2012 flare, dashed lines depict the displacement of the images over the 2012−2014 period.