As reported by @at2software, minor ticks are being hidden if there are not at least 2 major ticks present. This causes minor ticks to appear and disappear when zoomed-in to manual or custom major ticks.

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if ((majorTicks == null) || (majorTicks.Length < 2)) |
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return null; |
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if ((majorTicks == null) || (majorTicks.Length < 2)) |
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return null; |