Spells: Selo's Accelerando Not Receiving Drum Benefits

[Xebarsis]

While using a drum, if I cast Selo's Accelerando it lasts for 12 seconds and seems to work fine. However, if I wait for a second tic of Selo's Accelerando (not turning off the song) then it suddenly becomes an 18 second song which no longer receives the benefits of a drum. Therefore, once it turns into an 18 second song on the second tic my Bard slows down to the speed as if he didn't have a drum.