In plain-text romanization, the standard and expected spelling is “sensei.”
That’s the formal, conventional representation, especially for typing and learning.
Phonetically, in natural speech, the vowel often compresses toward a long /e/ sound, so you may hear something closer to sense or sensee depending on context and speaker.
In stylistic writing (e.g. light novels or dialogue), you might occasionally see phonetic renderings to reflect speech, but in formal or instructional contexts, “sensei” remains the correct and expected form.
In short:
• Orthography: sensei
• Phonetics: can vary in actual speech
• Stylistic writing: sometimes bends toward pronunciation
Different layers, different purposes.
I think this may mostly be a case of people talking past each other.
One side is focusing on orthographic convention (how it’s written and typed),
the other on phonetic realization (how it’s actually pronounced in speech).
Those aren’t contradictory claims — they’re just different layers of the same thing.