Elucidating the nature, strength, and siting of acid sites in zeolites is fundamental to fathom their reactivity and catalytic behavior. Despite decades of research, this endeavor remains a major challenge. Trimethylphosphine oxide (TMPO) has been proposed as a reliable probe molecule to study the acid properties of solid acid catalysts, allowing the identification of distinct Brønsted and Lewis acid sites and the assessment of Brønsted acid strengths. Recently, doubts have been raised regarding the assignment of the 31P NMR resonances of TMPO-loaded zeolites. Here, it is shown that a judicious control of TMPO loading combined with two-dimensional 1H-31P HETCOR solid-state NMR, DFT, and ab initio molecular dynamics (AIMD)-based computational modeling provides an unprecedented atomistic description of the host-guest and guest-guest interactions of TMPO molecules confined within HZSM-5 molecular-sized voids. 31P NMR resonances usually assigned to TMPO molecules interacting with Brønsted sites of different acid strength arise instead from both changes in the probe molecule confinement effects at ZSM-5 channel system and the formation of protonated TMPO dimers. Moreover, DFT/AIMD shows that the 1H and 31P NMR chemical shifts strongly depend on the siting of the framework aluminum atoms. This work overhauls the current interpretation of NMR spectra, raising important concerns about the widely accepted use of probe molecules for studying acid sites in zeolites.