The bug

After fishing forever, I have seen many many Frostwalker II books, but have never seen Frostwalker I. I have never found either in chests, but the world is not greatly explored. Admittedly, this could be due to chance, but I have acquired around 9 Frostwalker II books and never a Frostwalker I book, so I am starting to think this may be a bug with the fishing loot tables.

Update: my statistic for fishing is > 17,000.

Reproduction steps

1. Run this loot command in a repeating command block for several minutes.

   /loot spawn ~ ~1 ~ loot minecraft:gameplay/fishing/treasure
   

2. Now run the following to count all Frostwalker I books:

   /execute if entity @e[type=item,nbt={Item:{id:"minecraft:enchanted_book",Count:1b,tag:{StoredEnchantments:[{id:"minecraft:frost_walker",lvl:1s}]}}}]
   

3. Result: Test failed. There are no Frostwalker I books.