I have had the same issue for the past couple of years, and it drives me more crazy over time, as my accumulated old events list gets longer and longer so there are ever more obviously wrong suggestions. At this point, Calendar ALMOST ALWAYS suggests something wrong (eg "Haircut with Joe at 3pm" instead of "Haircut with Jane at 10am"), and it is easy to "fat finger"/accidentally accept the suggestions, which then overwrite the text and also overwrite the meeting time length.
This issue affects both the Calendar app on the Mac (High Sierra 10.13.5 and prior versions), and the Calendar app on iPhone/iPad (iOS 11.4 and prior versions).
Note: This is a DIFFERENT issue than deleting Location Suggestions, which is easy to do w/ existing settings on the Mac and iOS. It is also a DIFFERENT issue than the "Find Events in Other Apps" preference because it happens even when that preference is turned off. I THINK it is also not caused by the "Allow Search & Siri Suggestions?" preference because when I toggled that off, I still got the issue. I also can't find any iCloud preference that addresses this -- does anyone else know of one?
Note to Apple Support: There are SEVERAL old threads in the Apple Support Communities asking different variations of this question, with many incorrect suggested answers. As far as I could tell, NONE of the other threads have a satisfactory answer. Most of those threads are also closed to new comments, so here we go again, let's ask it all from scratch! Someone from Apple should really take a look at this and come up with an a fix!
p.s.: IN ADDITION, for the past few months, if you put what Calendar thinks is a time in the event description, Calendar will also incorrectly overwrite the event time I have already manually selected with the new, usually incorrect, time. (That's a stupid non-optional feature that Gmail has also had for several years, where it is just as poorly implemented.). Someone at Apple should make it clear how this unhelpful "helpful" feature can be turned off as well. Thanks for listening.
