I can't seem to use any flowers and many other types of plants (vines, etc) in the cauldrons.

The white &f colour code also doesn't seem to work, although this might only be because of the item type.

I'm on version 1.12.2, is there a workaround for this?

On 1.12 Bukkit used the old Material names. You can still find them on the internet if you look for them. You need to use these old names as ingredients.

Alternatively you can use the command /brew itemName to check what name a certain item uses.