Guild Choice math issues with invests?

Anyone else noticing weird math issues on Guild Choice only? For instance right now the Guild choice is the ironwood sawmill. The investment per tick for me is 18,000,000. So each tick should go up by that amount, which up to tick #6 it does, then at tick #7 the calculation gets all weird and goes up a lot.

Tick list I am seeing:

1 tick = 18,000,000

2 tick = 36,000,000

3 tick = 54,000,000

4 tick = 72,000,000

5 tick = 90,000,000

6 tick = 108,000,000

7 tick = 162,000,000 ( this really should be 126,000,000)

8 tick = 216, 000,000 ( this should be 144,000,000)

9 tick = 270,000,000 ( this should be 162,000,000)

10 tick = 324,000,000 ( this should be 180,000,000)

Now I checked the non-guild choice buildings and those are calculating correctly like they should be. So at tick #7 for some odd reason and beyond that the cost per tick goes from 18,000,000 to 54,000,000 which makes no sense what so ever. Anyone else having this issue?