Standard calculations of cosmological first-order phase transitions usually assume critical bubbles to nucleate on a homogeneous symmetric vacuum background. However, this assumption can fail in weak transitions, where thermal fluctuations trigger subcritical bubbles before the standard nucleation temperature $T_n$. Motivated by this possibility, we systematically examine whether the homogeneous nucleation background approximation is self-consistent. By evolving the Gelmini-Gleiser subcritical bubble kinetics and comparing it with the standard critical bubble nucleation picture, we identify the parameter regions in which the background becomes apparently mixed. A detailed scan of these regions shows that sizable subcritical volume fractions arise when the two phases are nearly degenerate at $T_n$, the potential barrier is low, the difference of free energy between the symmetric and broken phases is moderate and the transition strength is weak. Within this simplified phenomenological kinetic framework, this scan further calibrates a practical reference criterion, ${\log }_{10}{\widehat{f}}_{\xi }(T_n)\simeq -1.95$, for percent level subcritical bubble volume fractions. Parameter points above this boundary should be treated as mixed background candidates rather than as ordinary homogeneous bounce points.

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• Electroweak phase transition
• Electroweak symmetry breaking