Guidance on Spiritual Mentorship in Marriage

Salam alaikum, dear sisters and brothers. I'm in a bit of a confusing situation and need some Islamic perspective. My fiancé, who is generally a good and practicing man, has recently become very close to a spiritual guide (sheikh) and has taken bay'ah (an oath of allegiance) with him. Since then, he's started keeping many things private from me, which is not like him at all. He explains that his relationship with his sheikh is a special spiritual bond. The change in him has me worried. I feel like I'm seeing a different person, and with all the different interpretations and groups out there, I'm fearful we might be straying from the straight path. My core question is about this practice of bay'ah. When is it appropriate for someone to take it? What is its true purpose and context in our deen? And most importantly, how much should such a relationship impact one's marriage and daily life? I want to support his spirituality, but this secrecy is creating a lot of anxiety for me. Jazakum Allahu khayran for any advice.