That's not a bug, it's a feature.
You can only have 1 hero of any type on the battlefield at a time. When it comes to spawning time and you have a hero on the field already, the hero you whould have spawned is saved in a backup area until the one that is on the field dies. Then, as soon as the one on the field dies, the one from the backup will be used. It's been this way since I implemented the heroes way back in 1.50 or whatever.
The reason for this is that each player can only build 1 of the hero buildings, and if you had a 2v2 situation where 2 players were protoss, there would be no point in the 2nd player building the arbiter tribunal if it almost never resulted in a mothership being spawned on the field because the first one was still alive. In this case, the arbiter from the 2nd player will go into the holding pen until the arbiter on the field dies. This of course is still no 'gaurantee' that the 2nd arbiter tribunal is required, but at least it means it is much less likely to be wasted. Now 3v3 where 3 of the players are the same race, there's much less need for all 3 of them to build heroes, particularly the types that take a while to be killed: warp prism and medivac dropship.
Note that when it comes to spawning time, if you have a hero on the field and one in the backup holding pen, then the one on the field will be healed to 100% health, shields and for the mothership enough energy to cast 1 more stasis. This was added back in 2.30 or so.
@3FFA:
These changes don't nerf the reaper in any way, so if the reaper is already the surperior strategy vs zerg, then that'll continue to be the case. I think however the reaper is only great vs zergling, so if the enemy went lurkers or roaches instead then the reapers probably aren't the best. However Protoss (as first player spawning red or green) only have 1 really good initial spawn build, which is 3 gateways + optional pylons. Hopefully the improvements in immortals and dragoons in this latest version will expand their choices a little, but I wouldn't count on it, simply because those units don't have the DPS that zealots do and immortals are simply too slow when you're trying to capture the prison cell. Building 4 cyber cores could be a potential build but unfortunately it requires 1 gateway first, so you end up with 1 gateway and 3 cyber cores and it's just not that good because the enemy units are generally all small so the stalker only does 50% damage vs them.
Note that the total $ gained at the time of the first spawn is a little over $1,000. But Protoss has to get all buildings built before the 5 second mark on the clock or they don't spawn, so Protoss really only gets about $950 to spend which is not enough for 4 immortals + 1 pylon no matter how you stretch it.
I'll probably drop gateway price to $230, just to help separate it out from the immortals at $250 and make them a little more of a "cost+" investment, since they're a lot stronger vs terran now.
None.