Polarization amplitude in an 8°× 8°patch centred around the Galactic centre. The linear colour scale was chosen to demonstrate the low-resolution CO template residuals in the 2018 maps. The residuals are most pronounced at 100 GHz, where the CO corrections are largest, and absent at 143 GHz, where there is no CO correction needed. Since the 2015 maps are not corrected for bandpass mismatch, they do not display the same artefacts..