First off, make sure you take photos so that we can help troubleshoot! There are some troubleshooting steps we can take to diagnose what is happening. It is possible that this is a real result. 2,3-pentanedione and/or acetoin, which are vicinal diketones (VDK's) similar to 2,3-butanedione (diacetyl) sometimes show up as a dark circle, especially with certain types of yeast. You won't necessarily detect any diacetyl via sensory because 2,3-pentanedione and acetoin are not as potent as diacetyl. However, before we can say that, I want to make sure everything else about the test is running smoothly.
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