How's your hunger level? If appetite's staying low and holding your deficit isn't a struggle, dropping the amount could make sense, potentially even under the usual starting point. Then watch whether that helps, and creep upward very gradually from there. If you'd been running a steep deficit or already dropped significant weight, that could explain the tiredness, but this early on it sounds more medication-driven.
My guess is that fatigue traces back to the brain region tied to nausea and general malaise (working through those same receptors), typically a smaller problem on the newer dual-action drug compared to the single-pathway one, though reactions genuinely vary a lot person to person.
Had something comparable happen on the older medication myself, though mine felt more like mild illness than plain tiredness. Couldn't really stop it since it was suppressing my appetite and I was working hard to maintain a big loss, and despite tweaking small secondary doses constantly, it never fully cleared until I stopped that one and moved to the newer option. Most people do find the nausea and exhaustion ease up gradually over several months though.