Laelia (Cattleya?) purpata watering

[SouthPark]

If consistently the roots that are growing into the media survive, while the other ones didn't survive (turned black) ------ then that would be something to look into -- such as the degree of growth of the surviving root into the media, and how well connected to the media those particular roots are ----- things like that.

Also considering humidity and rate of change of temperatures between two significantly different temperature levels.

And even considering that different kinds of orchids, and even different individuals (depending on conditioning etc) will have their own response to significant change in conditions that they're not adapted or accustomed to.