Combining certain enchanted books on the anvil causes the second book to be erased.

Intentional. Some enchantments "conflict" with others.

Shouldn't it not combine instead?

No... they conflict. They can't be together.

What I think TS might mean, is that instead of allowing a book to be produced with the conflicting enchantments not added, perhaps it should produce nothing at all, showing the red x over the arrow, like it does when the enchantment is too expensive. Although .. then you couldn't combine a book with enchantments A and B, with another having C and D, to produce a book with A and D, when for instance B and C conflict, as I assume it could now.

Happens if you try to combine a tool with many enchantments. It won't let you have a lot of them, so it will not always include them all.

However it is now easy to accidentally lose a book because the Anvil lets you combine them even though it serves no purpose.

Would be fair if it didn't let you combine.

Yes, the voice of Ron Swanson is right.

I think there is some confusion here. In creative mode, you CAN combine enchanted books, to have multiple enchantments on one book. Therefore, what I describe in this bug report, would not be an issue, as that is the way the tool system behaves. (Conflicting enchants erase each other.)

However, the bug I am reporting is for survival mode. In survival mode you CAN'T combine enchanted books to produce books with multiple effects.

Therefore, the above combinations should not produce anything when placed on the anvil, as it is part of a feature not implemented for survival.

Unless we are meant to be able to combine books in survival for multi-enchant books? 🙂

If you can't enchant the extra conflicting enchantment, then why let you cheat yourself out of an enchantment book, which are already extremely hard to find?

I can second that there is in fact a bug with this, though what the bug is exactly is uncertain.

Essentially, while in Survival the only Enchanted Books that you can combine are invalid combinations, as such for Enchanted Books:

You can NOT combine Silk Touch I with Efficiency IV.
You can NOT combine Efficiency III with Efficiency III.
You CAN combine Sharpness I-V with Smite I-V, but all you'll get is whatever was in slot 1.
You CAN combine Protection III with Blast Protection IV, but same as last.
Etc.

So the bug is either the fact you are able to combine Enchanted Books at all while in Survival, or the fact you can ONLY combine invalid Enchanted Book sets while in survival, rather than all valid combinations.

Would it be possible to have a mod reopen this issue?

I don't think Dinnerbone fully understood the issue when he closed it.