Bosses keep their +3 level after changing from brutal to normal

I normally play on normal difficulty, but I changed it to Brutal to defeat the final boss of Act I and get the achievement. To my surprise, when I returned to normal difficulty, Vincent the Frostbringer and Christina the Sun Priestess were level 47, even though the V blood tab said they should be level 44. The problem did not occur with Beatrice the Tailor or higher tier bosses. Bosses did not have additional attacks as they did on the Brutal difficulty level, but they were stronger and harder to defeat.

Server was private PvE solo play, but the same bug occurred while I was playing with friend also on private PvE server, but he was the host, and we did similar thing with changing difficulty only for the final boss of Act I.