It is sticky and gummy
Updated
Why it happens anyway
Gummy jam is overcooking, and it happens to careful people because the signals that say keep going are stronger than the ones that say stop. A batch that looks thin in a hot pan is usually already there.
Because the doneness tests are for a set, and a preserve with whole fruit in it is harder to read than a jelly. The temptation is to keep going until it looks right in the pot, and in the pot it is hot and thin. It thickens as it cools, so the point that looks right hot is already past.
Trusting the test over the look
This is the failure that most rewards a measurement, because the eye is reading a hot product and the test is reading a cooled one. The two disagree in a predictable direction.
The published tests exist precisely so the decision is not made by eye at the stove. The refrigerator test is the one that answers this question: a small amount on a cold plate behaves the way the jar will behave, not the way the pot looks. It is on the refrigerator test.
Where it shows up
It is most obvious on a spoon and least obvious in a jar, which is why it often reaches a customer before anybody notices. Test a cooled sample rather than judging the pan, every batch, including the ones you are sure about.
In the jar, as something that will not leave the spoon, and on toast as something that tears bread. Both are repeat-purchase problems rather than complaints.