I tried reproducing the described problem, but I was unable to do so. I have attached the project for your reference.
From what I can guess the second linear axis (on the right) has a manual range. Perhaps when you add the second series, by mistake, you also set the VerticalAxis property of the firs series and it results in bars that are outside the manual range. You can debug to check whether the VerticalAxis of the first series is set or not to the VerticalAxis of the second series. If you still experience issues with this, I will ask that you send a modified version of the attached project that reproduces this so that we can investigate.
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