Multiple intriguing phenomena have recently been discovered in tetragonal Heusler compounds, where $D_{2d}$ symmetry sets a unique interplay between Dzyaloshinskii-Moriya (DM) and magnetic dipolar interactions. In the prototype $D_{2d}$ compound ${\mathrm{Mn}}_{1.4}\mathrm{PtSn}$, this has allowed the stabilization of exotic spin textures, such as first-reported antiskyrmions or elliptic Bloch-type skyrmions. Although less attention has so far been given to the low-field spiral state, this remains extremely interesting as a simplest phase scenario on which to investigate the complex hierarchy of magnetic interactions in this materials family. Here, via resonant small-angle soft x-ray scattering experiments on high-quality single crystals of ${\mathrm{Mn}}_{1.4}\mathrm{PtSn}$ at low temperatures, we evidence how the underlying $D_{2d}$ symmetry of the DM interaction in this material is reflected in its magnetic texture. Our studies reveal the existence of a novel and complex metastable phase, which possibly has a mixed character of both the Néel-type cycloid and the Bloch-type helix, that forms at low temperature in zero fields upon the in-plane field training. This hybrid spin-spiral structure has a remarkable tunability, allowing to tilt its orientation beyond high-symmetry crystallographic directions and control its spiral period. These results broaden the richness of the exotic magnetic phase diagram of Heusler $D_{2d}$ materials and extend their tunability, thus, enhancing a relevant playground for further fundamental explorations and potential applications in energy-saving technologies.

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• Helicoidal magnetic texture
• Skyrmions
• Noncentrosymmetric materials
• Topological materials
• Resonant elastic x-ray scattering
• Small-angle x-ray scattering