A comprehensive ${}^{31}\mathrm{P}$ nuclear magnetic resonance (NMR) study, combined with thermodynamic measurements and first-principles band-structure calculations, has been conducted to explore the ground state of the $S=5/2$ double trillium lattice antiferromagnet ${\mathrm{KSrFe}}_2{\left({\mathrm{PO}}_4\right)}_3$. Our experimental results indicate that the magnetic ground state is neither a conventional three-dimensional (3D) long-range order (LRO) nor a pure gapless spin-liquid state, as conjectured previously [Boya et al., APL Mater. 10, 101103 (2022)]. Specifically, the observation of a nearly field-independent NMR linewidth below $T^{*}=(3.5\pm 0.4)\mathrm{K}$, and a significant enhancement of spin-spin relaxation rate $1/T_2$ below $2T^{*}$ (where $T^{*}$ is the characteristic temperature identified from the magnetic susceptibility), indicate a complex magnetic ground state where spin freezing coexists with persistent dynamics. Furthermore, we argue that the lack of magnetic LRO and the persistence of strong magnetic fluctuations in ${\mathrm{KSrFe}}_2{\left({\mathrm{PO}}_4\right)}_3$ are unlikely to originate from intersite K/Sr disorder, rather they arise due to intrinsic magnetic frustration. Our findings position ${\mathrm{KSrFe}}_2{\left({\mathrm{PO}}_4\right)}_3$ into a broader family of geometrically frustrated magnets characterized by coexisting spin freezing and pronounced antiferromagnetic fluctuations, marking it as a promising platform for investigating exotic phenomena in 3D frustrated magnets.

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• Exotic phases of matter
• Frustrated magnetism
• Spin liquid
• Magnetic insulators
• Nuclear magnetic resonance