I do not know the cause, but if you have two or more stacks of the tome upgrade (common upgrades are 50% stronger) then common attack speed upgrades no longer have any effect.
Uncommon upgrades still work, and previously added common upgrades will work, but no new common attack speed upgrades have an effect.
I have tested it with normal and triple shot staves, and with 0-4 tome upgrades, and with 2, 3, or 4 tomes this bug is present with either staff.
Yeah, the first time I actually tested it (at first I didn't notice) I had ~1.5s/attack. After 5 common upgrades with 3 tomes I still had ~1.5s/attack. It should have been 37.5% faster for each at that point, which should have dropped it to about 0.14s/attack. And as soon as I got the uncommon one it went to ~1.1s/attack
Edit: Just to confirm for you, the 2.3 patch does not appear to have changed this bug. 0-1 tome still operates as it should, 2+ tomes cause common attack speed upgrades to have no effect, uncommon versions still work properly.
I did not expect it to have changed, just thought I should let you know after the patch as I am aware of how strangely code can behave sometimes...
