Unexpected bonus damage from bombardment unit on core province

Attacker:

• Comintern RA at 100% health
• Base attack of 3.2 / 2.0 / 1.4
• Located on a core hills province
• Adjusted attack of 5.5 / 3.5 / 2.4

Defender:

• Axis AC with 8.1 HP remaining
• Located on captured forest province
• No damage reduction bonuses

Expected result:

• RA inflicts ~3.5 damage to AC, which goes down to ~4.6 HP

Actual result:

• RA kills the AC, doing at least 8.1 HP damage in one shot
• That's at least 131% of additional, hidden bonus damage