Event currency always spawns at the same times, with max 6 spawned in the city simultaneously (if there are already 6 in your city, spawn will be skipped).
Spawn times in UTC (+- few seconds due to differences in server clocks):
0:38:25
1:14:25
1:44:24
2:38:25
3:38:25
4:30:25...
Ugh, was wrong. Not showing icon is a separate bug. Just got the same:
refreshing sometimes get it fixed, sometimes not. You can get it few times in a row.