Multiple intriguing phenomena have recently been discovered in tetragonal Heusler compounds, where D2d symmetry sets a unique interplay between Dzyaloshinskii-Moriya (DM) and magnetic dipolar interactions. In the prototype D2d compound Mn1.4PtSn, 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 Mn1.4PtSn at low temperatures, we evidence how the underlying D2d 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 D2d materials and extend their tunability, thus, enhancing a relevant playground for further fundamental explorations and potential applications in energy-saving technologies.