i was just having this problem. Resolved by following Wingman182's comment on this reddit post:
To summarise, do system options, display/selection, and change 'tangent edge display' to either phantom or removed.
Worked for me. It even contained the setting to the one part i wanted to change, and left the rest of the assembly alone. You might have to set it in the part design window (ie no assembly window) for this though.
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