iPhone Calendar search sometimes fails

My iPhone Calendar is mostly reliable. However, I noticed a search on a partial event name I did not find all events for the month; 3 events were missing out of 7 expected. When I cleared out of search, all 7 events were present. There seems to be nothing about their event names that would cause the search to miss. I tried closing the Calendar App and restarting my iPhone, as suggested, but the problem was still present. My iPhone has iOS 11.4.1.

• I also have an iPad Calendar on my iPad 3 with iOS 9.3.5; when I searched there, all 7 events were present.
• I replicate my calendar with Google Calendar; when I searched there, all 7 events were present.
• I replicate Google Calendar with my Thunderbird Calendar; when I searched there, only 2 of the 7 events were found.