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Tough to say. That’s why I would test at several other clear frequencies, to eliminate interference as the potential cause.
Grounds can also cause interference, depending on what you’re grounding to. Are you using any sort of ground with the ATU, or leaving it ungrounded. You could try both to see what happens.
You might also try another coax cable – maybe there’s something wrong with either your coax to the ATU, or the connectors.
Unfortunately, sometimes it’s just plain old trial and error that gets things resolved. Trying out different things may lead to the root of the cause, which could end ultimately up being just a defective ATU. And if you had another ATU, you could definitively say yes or no to that one.