I’m not sure if this is applicable as the issue appears to be in the original work.
The author revealed that Yahagi and Michelle were engaged in volume 3 part 1.
“… I’ll also introduce you as my fiancée.” That was right. We were finally engaged.
Later, in volume 4 part 9, Yahagi reveals that he plans on proposing that evening:
What was going to happen when I proposed to her after this?
If JNC has authorization, the plot hole could be resolved via modifying the 2nd quote to say that Yahagi is setting a wedding date.