Player tracking vests depend on signals from satellites, and those signals do not survive a roof. Covered stadiums therefore run a different positioning technology entirely.
The signal is weak before it reaches the ground
Positioning satellites transmit from a great distance, so the signal arriving at the surface is extremely faint. It works outdoors because nothing is in the way.
A stadium roof, a deck of upper seating or even a large overhang is enough to attenuate that signal below the level a receiver can use.
The receiver does not fail cleanly. It reports positions with growing error first, which is worse than reporting nothing because the data still looks usable.
Reflections are more damaging than blockage
Signals bouncing off structure arrive later than direct ones, and the receiver reads the delay as extra distance. Position drifts without any obvious symptom.
Inside a bowl, reflections come from many surfaces at once, so the error changes as a player moves between areas of the field.
Speed and acceleration are computed from successive positions, so a wandering position estimate produces sprints that never happened.
Local positioning replaces the satellites
Indoor venues install anchors around the field that communicate with the same tag a player wears, measuring distance from several fixed points.
Because the anchors are close and surveyed precisely, the resulting position is generally more accurate than the satellite-based alternative outdoors.
The cost is installation. Anchors must be mounted, powered, surveyed and maintained in each venue, which makes the system a building fixture rather than portable equipment.
The two systems do not produce identical numbers
Sampling rates, filtering and the way each system smooths a path all differ, so the same run yields slightly different peak speeds.
A squad alternating between covered and open venues therefore sees step changes in its data that reflect the venue rather than the players.
Staff handle this by recording which system produced each session and comparing within systems, which is the same discipline required across surfaces.
Why this matters more in the United States
A large share of major American venues are covered or partly covered, and roofs on retractable buildings may be open for one match and closed for the next.
That makes the tracking environment a per-match variable rather than a per-venue constant, which is an unusual complication by international standards.
Any analysis of workload across an American season that ignores which system supplied the numbers is comparing measurements taken with different instruments.