I can’t believe this hasn’t been fixed…but it also doesn’t look like anyone submitted a Github ticket for it…so I just did here: Support interface bottom layer not generated for tree supports · Issue #7728 · bambulab/BambuStudio · GitHub
Please comment on it to boost it if you want them to pay attention.
So just to clarify — did I really understand your bug report as a request to place the interface at the bottom? That’s… quite an interesting approach.
Let’s assume I’m using PETG to support PLA. According to your suggestion, the base would be PETG, then comes PLA as support, and then PETG again on top. That would fail faster than you can say ‘delamination’.
Just today I ran into the same issue I guess; Organic Tree support just “skipped” 4 bottom layers of support. Tried Hybrid & Strong and that does seem to generate it. Running the print with Hybrid now but would prefer to be able to safely use Organic without having to check using the preview if all is in place 
Your tone suggests to me that you think this approach makes no sense, but it’s already done in normal mode. It’s only failing to work in tree mode; still in May `26.
Fair point — if normal support mode already generates a bottom interface and tree mode does not, then yes, that is a slicer inconsistency and likely a valid bug report.
My objection is not that a bottom interface is conceptually impossible. My objection is specifically about treating a PLA/PETG release boundary as mechanically equivalent to a normal support connection, especially with tree supports.
PLA and PETG are commonly used as support-interface materials precisely because they do not bond strongly to each other. That is useful at the model/support contact surface, where clean separation is the goal. It is not automatically safe when that weak boundary becomes part of the load path for a tree support branch.
With normal/block supports, the contact area is usually larger and the structure is more laterally stable, so the slicer may get away with a bottom interface. Tree supports are different: narrow branches rely much more on local adhesion and structural continuity. Putting a deliberately weak PLA/PETG interface at the base of such a branch can easily become a failure plane.
So I agree that tree mode should probably expose the same bottom-interface behaviour consistently. But for PLA/PETG specifically, it should be treated as a controlled option, not as something that is always mechanically safe.