The X-ray Integral Field Unit (X-IFU) is an imaging spectrometer based on a large array of Transition Edge Sensors (TES) measured using Time Domain Multiplexing (TDM). For the development of a backup detector array, we have designed and realized a cryogenic test setup capable of measuring 9 detectors in a single cooldown under DC bias. We have used this setup to study a small selection of low aspect ratio TES designs, intended to have a low normal resistance suitable for TDM readout. In this work we show how the different designs are affected by magnetic fields. We do this by presenting the impact on the transition shape, detector integrated Noise Equivalent Power (NEP), and sensitivity of the energy scale calibration. We find, in agreement with previous studies, that reducing the width of the TES bilayer greatly improves the detector resilience to magnetic fields, potentially by several orders of magnitude.