Sorcerer [Cultist Background] Summon Duration Increase Not Applying

[Ser Shroom]

The cultist sorcerer background is documented as providing the following perk: -

Spell Modifications
Summon Creature duration increased by 2 rounds per level

I've done some time testing and it seems this bonus isn't applying. A cultist with GsF: conjuration gets the standard 6 rounds per level instead of what should be 8 rounds per level with this bonus.