I added some support filament in my AMS and attempted to print a balloon dog that I downloaded off of Thangs. I set the support interface to the bamboo support filament. If I had used bambu filament for the support base, it would have changed filaments over 600 times and takien 20 hours. But, when it printed it did not use the bambu support filament.
Has anyone else had issues or can tell me what I did wrong?
I have the same issue with both Bambu and Orca 1.5.x slicer versions. I opened a GitHub issue for Bambu Studio tonight for it.
I think it may be an AMS Sync thing. I saw a post about it demanding a particular color be in slot one of the AMS. The AMS logic is odd.
Try doing a Sync before choosing the filaments to use.
Using the support filament for the entirety of the supports is uneeded and wasteful (and $$$ 
).
I just did several prints with PLA in slot 4 and Support W in Slot 2 using interface only and it worked fine.
Silly Q I know, but are you on the latest Firmware and Studio/OrcaSlicer?
I did a resync before doing the slicing, but can try again. It showed all 4 filaments properly in the AMS listing in the slicer.
I am using latest firmwares, slicer versions, etc.
I still have my starter roll of support-w so not really going through it too quickly, but for the shapes I am doing I will need supports.
I can probably get by with regular filament if I really had to.
Yeah, I’m hording mine too! Well, I’m outta ideas. 
Yeah, I had support in slot 3 and the PLA I was using was in the second AMS, slot 7.
And my bamboo studio is up to date.
So it appears that if I change the support from tree back to normal, the support filament will work.
It seems that it will not work if you’re using tree supports.
I think the problem might be that in the default settings there is an empty layer on top between support and model. So there is no interface support, because support never touch the model. You have to reduce this top empty layer to zero (sorry, not able to look at the exact naming in Bambu studio right now)
Check the settings for Top Z Distance and Top Interface layers. Then check in the slicer if there actually is any material calculated for support.
EDIT: Also check if Top Interface Spacing is set to 0.
I received a response back from Bambu and it appears they have changed the slicer behavior in regard to Tree Support interfaces.
Previously, at least from what I recall as a newbie, when you changed the support interface from default only the base would use the default filament, and the tree (including attach points) would use the specified interface.
Now it appears the base and the tree are using the default filament and ONLY the attachment points of the tree to the model will use the specified interface filament.
This change was likely made to speed up printing by reducing the # of filament changes where there was no benefit in terms of model removal.
I have replied back to Bambu to confirm my understanding regarding the behavior change.
Honestly, I think my problem was that I did not restart bambu Studio. Once I closed it out and reopened it it seemed that everything was working fine.
Just a heads up -
I was just playing with Tree supports (OrcaSlicer). PLA and Support W. Changing the Support Interface only, not the Support Base.
Found if I chose Tree (auto) > Tree Slim, it would not do Interface with the Support W. All other modes would do it.
So, Interface Material is available for Trees, just not for Slim.