I was under the impression that private pensions were not generally ignored for IB or ESA, unless as Mrs J and Friendly Red said, you had been in receipt of IB for a long time. Ie, since well before the date when you first claimed it. Receipt of DLA or PIP for either component, and at any rate normally would not have anything to do with the disregard of a pension from another benefit.
You may need to contact the DWP to find out for certain. But if you have been overpaid IB as a result, you are going to need expert help in ensuring that this is correctly ascribed to have been caused by official error (given that you told them about the private pension when you first claimed) and that as such it should not be recovered from you (since you quite reasonably believed that the pension was ignored).
It isn’t always a straightforward matter to get these things sorted out. In your position therefore, I would contact the CAB and ask for welfare rights help in sorting it out. You can take all the various forms and letters you’ve received from the DWP to the CAB and hopefully you photocopied your original IB claim so can take that to an appointment too. They should be able to liaise with The DWP on your behalf.
It is of course, still possible that your IB and ESA claims have been correctly disregarding your private pension all along, but you should make sure that your claim is correct now that this question has arisen.
The CAB can also check your current PIP entitlement and help you to get a mandatory reconsideration if they believe your claim is being underpaid. But you need to get a move on with that because you only have a month from the date of their decision.