I think it was a bug.
You don't have to wait for standard quests to appear. They are always there.
It is a matter of knowing or guessing (or dumb luck) which items are quests and which are not.
For example if you start from scratch and train 435 of any troop type, you complete all 5 quests (10, 25, 50, 100, 250) for that tier even though they don't show in the quest suggestions. I did this for the tier 2 and tier 3 quests. Another interesting feature is that you don't have to train all of the troops in one go. So, for example, if you have completed the 10, 25, 50 and 100 quests then lost all of those troops, then you train 100 but 80 of them get killed, you only have to train another 150 to complete the 250 quest.
This doesn't apply above tier 3 because there are no standard quests for other tiers at present.
Also, it is not necessary to have 4 infirmaries at once, you can build then demolish then build. I did this.