Aditya, buttercream is an emulsion, and emulsions break when the fat gets warm enough to stop holding the water phase. Nothing about your technique is wrong. The ambient temperature is doing it.
Meera's rescue is correct and worth memorising, including the crucial detail that soupy and curdled are OPPOSITE problems - chill for soupy, gently warm for curdled.
Prevention in Indian summer: butter temperature matters more than anything, work in your coolest hours (in practice, early morning), keep the bowl away from sun and the oven. And seriously consider switching meringue base for the hot months - Swiss and Italian meringue buttercreams are meaningfully more heat-stable than American, and Italian in particular survives a delivery run far better.
If the cake also has to survive transport and a non-AC room at the other end, that's a formulation decision, not a technique one. Choosing the right buttercream for the season and the delivery is part of the job here in a way it just isn't in the European recipes most of us learned from.
