Export priorities not being respected

Issue description

I have a bucket attached to my smelter for slag with an export priority of 3. Dedicated dumping zones are set up to allow slag dumping, but are NOT explicitly linked by adv. logistics.

My trucks are ignoring the export priority, and instead are exporting dirt and stone from a mine - these are set to priority 14.

Completely ignoring this:

In favour of this:

Reproduction steps

1. Load attached save
2. Observe slag bucket attached to iron smelters ('Slag Export')
3. Also observe dirt/stone export buckets at coal mine
4. Trucks are mostly ignoring slag output and instead emptying the lower priority buckets at the coal mine

Game version and build number

v0.8.5 (b598)

Operating system

Windows

Attachments

0009 - Something funky.save

I have been playing with Power for belts and storage set to 'Always Consume'. I started to suspect that because the buckets have two separate priority settings - one for 'general priority' and the other for export - whether this may have been affecting the truck export priorities. I tried making some changes and didn't notice any difference. I have now changed the difficulty setting to 'Consume if can' and now seeing the trucks respect the export priority correctly.