Sounds like either a bug, or it is similar to the way that summoning magic skill is related to Blood Pact: Ward durations. With Blood Pact: Ward, what matters isn't your summoning magic skill itself, but rather how far it is over the cap. If the ability is based on some hidden "skill" in the same way the Blood Pact: Ward is, which in turn is set to level times a flat constant, it would explain what you are seeing. At low levels, the difference in skill caps between adjacent levels is small, so the flat constant multiplier is overrunning it and ramping your duration way up; as your level increases, however, the skill cap increase per level is larger than the multiplier, causing your "skill" to fall behind. Eventually it drops back under the cap and you start seeing flat minimum durations again.
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