I noticed this behavior only when Inventory Profiles Next is installed, I think because of its autofill feature:
When cycling multiple times in a row with tab, the main held item slot does not cycle if it's pressed too fast again, while all other slots will successfully cycle. This can cause the inventory to get really disorganized if you forget to wait between cycling.

I would love to see compatibility with IPN, both of these mods together make the perfect inventory & hotbar experience. Thanks!

Thanks for reporting and liking this mod!
I have noticed its bad compatibility with IPN, and it might be hard to deal with. However, I shall fix this when I figure out how IPN works.