Aditya, buttercream is an emulsion, and emulsions break when the fat gets warm enough to stop holding the water phase in suspension. Nothing about your technique is wrong — the ambient temperature is doing it.
Meera's rescue is correct and worth memorising, including the important detail that soupy and curdled are opposite problems with opposite fixes. Chill for soupy, gently warm for curdled.
On prevention in Indian summer conditions: butter temperature matters more than anything else, and the target is a genuine range rather than a vague sense of soft. Work in your coolest hours, which in practice means early morning. Keep your bowl and your finished buttercream out of any direct sun or near the oven. And consider switching meringue base for the hot months — Swiss and Italian meringue buttercreams are meaningfully more heat-stable than American, and Italian in particular holds up considerably better on a delivery run.
If the cake also has to survive transport and a room without air conditioning at the other end, that is a formulation decision, not a technique one. Choosing the right buttercream for the season and the delivery conditions is part of the job here in a way it simply is not in the European recipes most of us learned from.
