@Riot Question Regarding Spellblade

I was under the impression that when using different items with the same spellblade passive the best traits from each spellblade item would be combined.

While fooling around and using Runeglaive + Iceborn Gauntlets, I noticed that the majority of the time I did not receive the AOE slow from Iceborn on proc... though I sometimes intermittently did.

Shouldn't Iceborn + Runeglaive basically give you this on proc: 125% Base AD + 30% AP (Converted to magic damage) Damage applied in AOE w/ an AOE slow

With the upcoming changes to Runeglaive, will the AOE slow be added back to this combination? Or was it a glitch that the slow wasn't applying in the first place?