I did a search for "Death knight bug", "Horn of winter bug", "frost presence bug", and nothing came up, so sorry if this is a repost.
For this to work, you need to have the talent "Bladed Armor" (Bladed Armor - Spell - World of Warcraft). This bug works by using Horn of Winter and frost presence.
Do the following:
1. Activate frost presence
2. Cast horn of winter
3. Switch to either blood or unholy presence
4. You will retain the AP bonus from bladed armor working with frost presence.
Because frost presence adds 60% armor, it buffs bladed armor, and horn of winter prevents the AP from being lost while changing presences. In T8 gear, it adds roughly 300 ap, but you have to repeat the above steps whenever horn wears off to retain the effect.