Just like Gemini, Sagittarius also is a dual sign and here 7th lord is Mercury which is badhak.
Sagittarius wants to adopt the personality of a teacher, or guru and wants to live life with with own morals and ethics.
They are straightforward, goal-oriented and don’t like the intellectual/logical part of interaction…they want to decide what is right (highest dharma sign) ethical way.
All Gemini traits of multi-focus, over curiosity, and duality of other people can be the badhak for them.
Mercuy’s connection with gulika or khara planets can become the obstruction during dasha /antardasha of the planets… The nature of obstructions depends upon the house of gulika and Mercury.
Mercury in dusthana houses in the divisional chart is good and will reduce the bad impact.