I've seen this in live worlds, but it doesn't gives the shown extra mana. Just the tooltip seems to be wrong.
After a relaod the tooltip is correct (without additional mana).
How can this be a bug ? Like I wrote before: this requires a lot of programming to make it happen. If that's a bug then your programmers are doing a very bad job !
This spell should giving 5% only to cultural bonus(like it description says), so just think... is entire cultural bonus giving u addicional mana? If yes u should getting f.e ~125/150% additional manna whole time even with out EE. With this case programmers just made mistake with code and forgot to restrain bonus only to cultural bonus which giving addicional coins and supplise only. ....ofcours IMO