We report ultrasound velocity measurements on an ultraclean ${\mathrm{UTe}}_2$ single crystal with $T_c>2\mathrm{K}$ for $H\parallel b$, performed up to 18 T and down to 0.33 K. The measurements provide the missing bulk thermodynamic evidence for an additional high-field phase boundary near ${\mu }_{0}H\sim 14–15\mathrm{T}$. A distinct, nonhysteretic anomaly in the longitudinal $C_{33}$ mode, together with a weaker response in $C_{44}$, no resolvable anomaly in $C_{55}$, and a coincident kink in transverse magnetostriction, reveals a symmetry-selective coupling to lattice strain. This mode selectivity places constraints on the symmetry of the field-induced superconducting component. The phase line remains nearly field constant near 14 T and meets three other phase boundaries at a tetracritical point near 13.5 T and 1.25 K. The results complete the local tetracritical topology of the $H\text{−}T$ phase diagram and support field-induced multicomponent superconductivity in ${\mathrm{UTe}}_2$.

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• Critical field
• Phase diagrams
• Second order phase transitions
• Superconducting phase transition
• Superconductivity
• Heavy-fermion systems
• Unconventional superconductors
• Ultrasound techniques