Not sure if that's VA or VS that causes the problem but its damn annoying and happens always. After selecting some lines and pressing { to enclose the selection in curly brackets, the damn VS suggestion list pops up now.
That's really odd, and no sign of this happening here. Can you please try in a new, default C# solution, just to see if this is somehow solution specific.
Can you please export your VA and IDE settings and send them to me:
VA Options -> Performance -> Export Settings IDE tools menu -> Import and Export Settings -> Export selected environment settings
I can then import them here and see if I can reproduce the problem. Please send me the files via email:
My guess is that its a new "feature" of VS 17.5, has never happened before. But then again, I did update VA+VS at the same time, so I cant blame one or the other.