New mineshaft for each desired ore?

[mattregul]

Describe the bug Around year 6 of a large fortress (and a DF & DF-AI reload), the AI decided to dig many new mineshafts for deep treasures.

• It looks like there is one new shaft for each z-level we'd like to reach.
• Perhaps it's because the AI is avoiding water/caverns/etc on the way down, looking to get a straight line to the goal?
• In the early game, the AI was reusing existing tunnels to reach 10-20's range
• These new shafts end in a range from 35 to 80's z-level.

Expected behavior Use pre-existing shafts more often? Perhaps bigger shafts closer to the map edge?

Visual reference

ai version:

Dwarf Fortress 0.44.12
  Windows 64-bit
df-ai 0.44.12-r1-update1-8-g7b306e26 (Build ID: df-ai-pre-0.44.12-r1-180810000)
  code 7b306e2669d0c4fd933a73e360e5f9f0a7ba7ee6
DFHack 0.44.12-r1-0-gc2a64b35 (Build ID: df-ai-pre-0.44.12-r1-180810000)
  library c2a64b3573abbee549761f5c1c2626eec7a6e81c
  structures 23500e4e9bd1885365d0a2ef1746c321c1dd5094