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Are you sure it's a bug? I've had my mercenary company go independent via event, after which I could form a new one. If the event just happened to fire at the time the ruler died, I don't see how it's a bug. OTOH, if the company went independent solely upon the ruler's death without an event firing, then yeah, it's a bug. I can't tell from the OP which happened.
What's more likely is that the person who succeeded the previous ruler already had a mercenary company, and rather than inherit the previous ruler's company, they just kept their own.