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If the province's revolt chance is low, the support may not create an actual revolt, but will increase the odds of it happening in the near future. In some cases, if the province is fairly stable, it may not increase the risk above 0, so a revolt will cannot happen. If the chance is already high, the added support is very likely to cause an immediate revolt.

In a couple of campaigns, I had one or two countries spamming spies to foment revolts every few months, and in one game I had a particularly annoying OPM hit me with another attempt EVERY SINGLE MONTH for several years (for no rational reason), and I couldn't do anything about it because they were allied to a major power and guaranteed by the Emperor of the HRE. ONE of those attempts (out of somewhere around 30) caused an actual revolt, because with +3 Stability, 100 Legitimacy, and mostly core provinces, the chance of a revolt was still well into negative numbers in all but a few cases.

Not a bug, as far as I can tell.