So yeah, two problems here. The first is that I got dragged into a war with the Iztahuacans (Red) in aid of the Halls of Time province, which said they would join my empire if I defended them from an impending Iztahuacan invasion in ten years time. Together we managed to wipe the Iztahuacans clean off the map before the deadline, their entire population slain or captured by the time my warriors entered their final deserted province. Despite this, the game does not cancel the event trigger and still spawns close to 200 Iztahuacan soldiers to storm the Halls of Time, which by now is deep within my own conquered Atlan (Blue) territory. I can hold them off, but I should not have to.
The other problem is that the Teotitlans (Cyan) to the north now seem intent to declare war on me. I did not even know where they were on the map, as the game colour codes them to look almost identical to Neutral (Beige) non-aligned provinces like the Wheel of Time once were before they decided to throw their lot in with me. I may be red-green colour blind, but even my brother with full colour vision had to squint to make out the difference. Raising the saturation on the image makes all seven of the Teotitlan provinces become apparent, but I am calling bull on all of this.