I was made to believe concentrated tea is more potent than dilute tea, as it's absorbed more efficiently.

Usually I take harmalas and follow them with leaf tea (chacruna or chaliponga), which I concentrate so that a serving is between 5 - 10 ml.

However, recently I've also drunk unreduced tea while it was being reduced, on two different occasions (different brews). The volumes were calculated accordingly and ended up being closer to one cup.

On both occasions I was surprised how strong the tea turned out. After subsequently reducing the tea to its final, high concentration, it was perhaps half as strong.

I wonder what's going on here. Is dilute tea absorbed better? Does volume reduction reduce potency, e.g. by destroying the alkaloids or trapping them in solids and making them less bioavailable?

Did you filter the brew after reducing it? If you did, thats prob why you lost some alkaloids. Also the more concentrated, the more can get lost in little spills and containers and what not.

That being said, Im not sure if the absorption and bioavailability changes with concentration. I'd have thought it wouldn't, at least not until you make it so thick that it takes a long time absorbing (for example cact resin takes longer to absorb than tea so it doesnt peak as much ime). and if it did make a difference, I wouldnt have expected it to be half as much, thats huge...

Are the comparisons you are making with the exact same batch and all other variables kept equal (such as how you brewed it) ?

It's with the same product. The first comparison was with the same batch (i.e. batch A unreduced vs batch A reduced). The second one is batch B unreduced vs batch A reduced, once batch B has been reduced I'll compare batch B unreduced to that.