I seem to have had precisely the same problem occur with "Talisman of the Unconquerable". I haven't seen the drinking horn yet, but is the effect meant to multiply focus gains?
Talisman's Effect in its description reads: "+2 Intellect while Endurance above 50%, x1 Focus Gain while Endurance above 50%" When examined from the character status screen instead of the inventory, the effect becomes: "x1.25 Focus Gain while Endurance above 50%, +2 Intellect while Endurance above 50%"
Copied the test in the OP with the talisman. I have no blunderbuss yet, so I used a saber instea and a paralyzed dark spore instead of a lion. My Test adventurer: TEST generated the following results:
(No amulet equipped): 2 hits, 1 crit TEST goes from 15 to 25 focus.
(Amulet equipped): 3 hits, TEST goes from 15 to 17 focus
(Amulet equipped and immediately removed): 1 hit, 1 crit, 2 more hits, TEST goes from 15 to 17 focus.
(Amulet equipped and immediately removed, character goes on to fight a second spore): hit, crit, hit TEST stays at 15 throughout
(TEST is allowed to level up to 2, purchasing draining whip after last trial. The amulet is not reequipped after leveling up): 1 crit, 1 hit TEST goes from 20 to 19 focus.
I have not yet seen the drinking horn of moderation in game, is the effect a focus multiplier like the amulet? And if so, has anyone obtained the "Brîshalgwin Mindmarker"? If so, does the bug repeat with that? The item is listed on the wiki as another focus gain multiplier and I'm wondering if the problem is universal to all of them.