It is plausible it is showing on only one report because the others aren't current. Sometimes a debt only appears on one or two of your credit bureau reports and not the other one. It can still negatively impact credit scores.
Collection agencies don't always report to all the bureaus as many people will pay erroneous debts as soon as they notice them. They make loads of money from people not checking and questioning the validity of collection accounts.
Would I personally work towards validating the debt and pursuing removal? Yes I would. Especially if it paid with insurance coverage, or is a potential an error.
It is completely conceivable it could be off base debt obligation, yet I truly can't address your own obligations and what could be invalid debts for you.
If the debt has any potential in being invalid why would you leave it on any of your credit reports? Collection accounts have negative impacts on credit scores no matter if only on one bureau report or all. You are out nothing but time in trying to get this debt collection removed.