It is strange to me that the English phrase "under control" would translate literally into Spanish. After all, in the phrase "under control" we are not talking about something being below something else. I have to wonder if the phrase "bajo control" is just a "spanishization" (to coin a word) of the English phrase? Does anyone know?
My three-inch thick very thorough Spanish-English dictionary lists "bajo control" as a Spanish phrase that translates into the English "under control" so I believe it's legit. Other similar legitimate phrases it lists are bajo los efectos de drogas (under the influence of drugs) and bajo la de tutela de (under the guardianship of), neither of which involve something being literally "below something else" physically, so it seems that in Spanish it has a similar idiomatic usage as in English.
A quick search finds numerous examples of "bajo construcción". One example I pulled out of a current news story: "Para esta semana, el gobierno había completado 273 kilómetros (170 millas) de muro fronterizo y están bajo construcción aproximadamente 290 kilómetros (180 millas) más."