For a few times now, I cannot seem to place a pillar on 1 of the hex tiles around my city. It shows up red on my city and district, but every now and then it is yellow on every other exploited tile except that one particular one. I thought it might be because it has an anomaly - nope, no anomaly. Next to an anomaly - nope, next to nothin'. Compared that tile with every other tile that I can place the pillar on - no visible difference.

Is this a bug or am I missing something here? This random cannot-place-pillar-on-empty-tile problem makes it difficult for me to play AM sometimes... If I build two cities next to each other then find out the pillar cannot be placed at the sweet spot where both cities can benefit from the pillar, that just royally wasted my plans.

Edit: here is the image, I forgot I alt-tabbed (Win + D since I'm on Win10) to ask this question...

Edit 2: Never mind I found out the reason. It's because I had a Borough Street planned to be built there. This makes sense now; the pillar lasts for 10 turns; effectively it is a "building that lasts 10 turns", so it cannot be placed on a tile that might sprout a building within those 10 turns.