  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 Mn$_{1.4}$PtSn, this has allowed the stabilization of exotic
spin textures such as first-reported anti-skyrmions or elliptic Bloch-type
skyrmions. While 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 Mn$_{1.4}$PtSn at low temperatures, we evidence
how the underlying $D_{2d}$ symmetry of the DMI 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
N\'{e}el-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 reachness of Heusler $D_{2d}$ materials exotic magnetic
phase diagram and extend its tunability, thus enhancing a relevant playground
for further fundamental explorations and potential applications in energy
saving technologies.
