[Oathbreaker] Can't cast Signature Spell (Show and Tell) even if Oathbreaker (Rowan, Scholar of Sparks) is in play

Exactly as mentioned in the title.
I haven't tried casting the Signature Spell with the other side of the Oathbreaker (Will, Scholar of Frost) in play.

Start by reproducing the reported case with Oathbreaker (Rowan, Scholar of Sparks) in play and attempting to cast Show and Tell. Trace the signature-spell handling from that reproduction, then verify that the spell can be cast with Rowan in play; testing the Will, Scholar of Frost side is also suggested by the report.

Written by the indexing model from the issue text.