I noticed that my main character (cipher) suddenly stopped being able to generate focus, and tested until I found the reason, namely the "Drinking Horn of Moderation" neck slot item.
To test:
I created a new cipher adventurer ("the character" below), and saved the game.
I loaded the saved game and equipped the character with a blunderbuss and nothing else, and attacked a paralyzed lion.
2 grazes, 3 hit, 1 crit, grazes at 6~ hits and crits around 12.
Result: went from 15 to 29 focus. This is what is expected.
Reloaded the saved game, equipped blunderbuss and horn of moderation, exited inventory screen (to let the buff happen), entered inventory screen, unequipped horn, and then attacked the same, paralyzed lion.
2 grazes, 3 hit, 1 crit, grazes around ~7, hits and crits around 11.
Result: went from 15 to 18 focus. Same result if you leave the horn equipped.
Reloaded the saved game, equipped horn and blunderbuss, got character knocked out, won the fight, and then used the character to attack a paralyzed lion.
3 grazes, 3 hits, grazes around ~7 and hits around 10.
Result: went from 15 to 14 focus. All attacks made after this one leave focus unchanged. Focus still refills to half after each fight ends.
This is irrevocable and persists through save/load, and also stops draining whip from working entirely, if you level up to 2 and take it and redo the tests with it.