Recently, an "infinite emerald cycle" was fixed in Librarian trades. ie: one could trade a book to a Librarian for 1 emerald each, and could also buy a bookshelf (which can be broken to obtain three books) from a Librarian for 1 emerald each. By this method, one could turn a single emerald into infinite emeralds, using the same two trades, sometimes using a single librarian.

This was "corrected" in 1.15.

However, there is still another "infinite emerald" cycle. This one is a little bit more complex, as it requires both a Cleric and a Librarian:

Clerics can buy Glass Bottles for 1 emerald each.

Librarians can sell glass, up to 4 for 1 emerald.

3 Glass can be crafted into 3 glass bottles.

So, the previous "fixed" cycle was: 1 Emerald -> 1 Bookshelf -> 3 Books -> 3 Emeralds

the new cycle is: 1 Emerald -> 4 Glass -> 3 Bottles -> 3 Emeralds

As it's between two different villagers, maybe it's not a problem, but I suspect the intent is to not allow "infinite emerald cycles" without any resource-gathering by the player.

If this is fixed, I suggest requiring a minimum of 4 bottles to get 1 emerald. I really don't want to see the glass trade nerfed.