A phase transition changes a substance's physical state entirely, solid to liquid, liquid to gas, and it requires a genuinely fixed amount of extra energy on top of the ordinary heat that simply raises a substance's temperature. That's exactly why a pot of water at a full, rolling boil holds steady right at its boiling point rather than continuing to climb hotter as more heat is added: all of that added heat is being spent breaking the molecular bonds holding water in its liquid state, converting liquid to vapour, not raising the water's temperature any further.
Ordinary heating and phase-change heating do genuinely different jobs
Heating a substance that isn't undergoing a phase change raises its temperature directly, speeding up its molecules' average motion. Heating a substance that's actively transitioning between phases instead spends that same energy breaking or forming the intermolecular bonds separating one phase from another, water molecules held together in a liquid versus moving freely as a gas, and that energy goes entirely into the phase change itself, with none of it available to raise temperature further until the transition is fully complete.
This is exactly why a substance's temperature plateaus during a phase change
Because all the heat energy added during an active phase transition is being consumed by the transition itself, a substance's temperature genuinely plateaus, holding constant, for as long as the phase change is still under way, resuming its climb only once every last bit of the substance has finished converting to the new phase. This is precisely why a pot of boiling water stays at its boiling point no matter how vigorously it's actually boiling, and why ice melting in a glass stays at its melting point the entire time solid ice and liquid water are both still present together.
What we're still unsure about
That a substance's temperature plateaus during an active phase transition, with added heat converted entirely into the phase change rather than a temperature rise, is extremely well established, thoroughly confirmed thermodynamics that's been measured precisely for well over a century. What's more genuinely a matter of specialised, ongoing research is exactly how phase transitions behave at extreme pressures and temperatures, or for genuinely complex mixtures rather than pure single substances, since the simple, clean plateau behaviour water shows can become considerably more complicated for mixtures transitioning over a range of temperatures rather than one fixed point, and physicists and chemists continue to study exactly how those more complicated phase behaviours actually unfold.
This sits inside Phase Transitions, one of seven topics in Thermodynamics, one of five domains in Physics, one of seventeen subjects the app can quiz you on.