This might help: https://creativepro.com/arranging-numbered-lists-separate-frames.php

It’s an excellent question and not immediately obvious. However, the main reason not to use Auto leading in InDesign is that each character has its own leading. So if you make a single character in the middle of a paragraph larger (for example 14 point instead of 12), then the leading changes for the entire line. Auto leading means “use whatever the leading is for the largest character on that line.”

Yes, the opposite of “auto leading” is not to turn it off, but rather just to apply “absolute” leading — meaning, an actual leading value, like 15 pt.

But Ari’s point about a leading that would change appropriately as the text gets larger is a good one. QuarkXPress has a % leading feature, which is kind of like Auto. But in XPress, leading is always the same throughout the paragraph. (You can change that in ID, though.)

More on that here: https://creativepro.com/reigning-in-rogue-leading.php

My guess it’s something like this: https://creativepro.com/eliminating-ydb-yucky-discolored-box-syndrome.php

Hm. And it’s not letting you? Very strange. Please contact magazine@indesignsecrets.com and Tammy will get back to you asap.

I don’t think I have a good answer for you.

Make sure that there is None character style applied in the text frame before you create the index.
Perhaps recreate the original template by exporting to IDML and then opening that IDML and then generate the index?

Or, if it keeps happening, you may just have to apply a find/change to search for all that character style and set it to None (in the Find/Change dialog box).