With unique advantages of high sensitivity, no-contact, and non-destructiveness, magneto-electroluminescence (MEL) is usually employed as an effective detection tool to visualize the microscopic mechanisms of excited states existed in organic light-emitting diodes (OLEDs) because their evolution channels of many spin-pair states in OLEDs have the fingerprint MEL line-shapes even with opposite signs. The recently-published MEL results have demonstrated the existence of high-level reverse intersystem crossing process (HL-RISC, S<sub>1,Rub</sub> ← T<sub>2, Rub</sub>) of high-lying triplet excitons (T<sub>2, Rub</sub>) in Rubrene when Rubrene with a typical content of several percent is doped into a host with high triplet exciton energy and there are also no any energy loss channels of triplet excitons from charge-carrier transporting layers. Furthermore, this HL-RISC process can considerably boost the efficiency and brightness of OLEDs operated at room temperature, for example, high external quantum efficiency up to 16.1% and ten thousands of brightness have been achieved in Rubrene-doped OLEDs with a co-host of exciplex. Herein, surprisingly, in the pure Rubrene-based OLEDs (i.e., the pure Rubrene film is used as an emissive layer) with no any energy loss channels of triplet excitons from charge-carrier transporting layers, only strong singlet fission (S<sub>1</sub><sub>, Rub</sub>+S<sub>0</sub><sub>, Rub</sub> → T<sub>1</sub><sub>, Rub</sub>+T<sub>1</sub><sub>, Rub</sub>) processes are detected at room temperature, but this HL-RISC process is not observed. Moreover, even the most usual evolution process of intersystem crossing of polaron-pair (ISC, PP<sub>1</sub> → PP<sub>3</sub>) cannot be observed in this pure Rubrene-based OLEDs, where the polaron-pair are generated from the recombination of the injected electrons and holes in the pure Rubrene emissive layer. To determine the cause of the underlying physical mechanism behind this abnormal and fascinating experimental phenomena, two kinds of devices with pure Rubrene and 5% Rubrene-dopant as emissive layers are fabricated and their current- and temperature- dependent MEL responses are systematically investigated. Via comparing and analyzing thes tremendously different MEL curves of these two types of devices, we find that the positive Lorentzian MEL curves induced from <i>B</i>-mediated ISC of polaron-pair just completely cancel out the negative Lorentzian MEL curves induced from <i>B</i>-mediated HL-RISC process of T<sub>2, Rub</sub> excitons. Note that such an abnormal and coincidental experiment phenomenon is the physical mechanism why ISC and HL-RISC processes cannot be observed simultaneously in the pure Rubrene-based OLEDs, and this phenomenon has never been observed in the literature. Clearly, this work has further deepened our understanding of some unique microscopic processes and physical phenomena in organic semiconductor "star" material of Rubrene (such as the energy resonance between 2T<sub>1</sub> and S<sub>1</sub> and the energy approach between T<sub>2</sub> and S<sub>1</sub>).