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One of the most promising tracers of the Galactic potential in the haloregion are stellar streams. However, individual stream fits can be limited bysystematic biases. To study these individual stream systematics, we fit streamsin Milky Way-like galaxies from FIRE cosmological galaxy formation simulationswith an analytic gravitational potential by maximizing the clustering of streamstars in action space. We show that for coherent streams the quality of theconstraints depends on the orbital phase of the observed stream stars, despitethe fact that the phase information is discarded in action-clustering methods.Streams on intermediate phases give the most accurate results, whereaspericentre streams can be highly biased. This behaviour is tied to the amountof correlation present between positions and momenta in each stream's data:weak correlation in pericentre streams prohibits efficient differentiationbetween potentials, while strong correlation in intermediate streams promotesit. Although simultaneous fitting of multiple streams is generally prescribedas the remedy to combat individual stream biases, we find that combiningmultiple pericentric streams is not enough to yield a bias-free result. Wefinally show that adopting the two-component St\"ackel model does notfundamentally induce a biased mass estimate. With our full data set of twomulti-wrap streams, we recovered the true rotation curve of the simulatedgalaxy within $12\%$ over the entire range of radii covered by our set of stars(10 - 176 kpc) and within $6.5\%$ between the 5 and 95-percentile distancerange (23 - 109 kpc).

Orbital phase-driven biases in Galactic mass constraints from stellar streams