If the province is landlocked (which is common when the AI creates border gore), the rebels will never teleport to siege other provinces and will just sit there. The AI will never accept the rebel demands, so the rebels will sit there forever. The rebels in the screenshot have been sitting there for 50+ years.
The teleport mechanic is extremely buggy. The rebels wont teleport past a strait either, so you see Danish rebels walking through Northern Germany to reach the other side of the strait, and obviously they never make it there since they will get killed along the way.
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