What environmental factor might explain why propellant system leaks manifested only after reaching orbit?

Extreme temperature swings causing materials to expand and contract repeatedly

Engineers considered the unique stresses of on-orbit operations as a potential trigger for the manifested leaks. Spacecraft propulsion systems are subjected to intense and rapid changes in temperature—extreme swings between direct sunlight and shadow—which causes the materials, such as seals within the propellant lines, to repeatedly expand and contract. An existing small anomaly, perhaps a seal compromised slightly during the stresses of ascent or pre-launch processing, might only become a measurable leak once the system has been fully pressurized and subjected to the vacuum and thermal extremes of long-duration flight in space, highlighting the challenge of certifying new human-rated systems against real-world conditions.