It won't stop if it's ventilated with outdoor ambient air:

40C air can hold 51 g of water per m3 of air. 60C air can hold 130 g of water per m3 of air [1]. The curve is exponential.

So, it works as long as the transformer is hotter than ambient air, even at the most humidest (100% RH). The transformer's heat will drop the relative humidity of the air near its surface, and the heated air can absorb more water again.

If the humidity is below 100% RH, what changes is that the evaporating water could cool it to below ambient air temperature, same effect as in swamp coolers.

[1] https://www.engineeringtoolbox.com/maximum-moisture-content-...