I am not sure how the reduction to THH would occur during the boiling of the ayahuasca, perhaps it is being reduced by something in the plant itself, a protein or enzyme (reductases).

Indoles can oxidize molecules through a similar mechanism I posted above. In particular the beta-carbolines, the methoxy group pyrrole and amine nitrogens can donate electrons from their lone pairs and distribute the density by resonance. There must be something that can pick up those readily available electrons to break the pi-bond, but it looks like it would be energetically favored.

Formates are known reducing agents in vivo, catalyzed by an enzyme they are decarboxylated to form CO2 and hydride which acts as a reducing agent. Interestingly enough to reduce iminium ions! Of course, boiling solution obviously denatures proteins and enzymes, but I suspect something of this nature is happening.

Here is a link: Formic acid

To answer your question, THH becomes racemic in vitro because when it is in aqueuous solution such as in ayahuasca brew, it is protonated. The epimerization mechanism can occur to give you both enantiomers. The plant (in vivo) seems to produce the d-isomer selectively.