If you run simulation enough, you may encounter this kind of weird issue. Waveform shows all the inputs to a cell is stable but cell’s output toggles. Likely it is because there is some glitch on cell’s inputs but you don’t see it in fsdb waveform. Simulation tool by default doesn’t dump glitch to fsdb and this is to reduce fsdb file size. You need to enable glitch dump to fsdb and rerun sim. Then you should be able to see the glitch.
Below is some detail how to enable glitch dump for VCS and Verdi.
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