Maybe it was an app having issues running on this specific firmware and an update fix it.
Thats possible, but there was a noticeable "to the minute" change when I updated to 11.4.1 Beta 4.
As a full disclosure, when I first upgraded to 11.4 I went through 2 stages: Stage 1 was horrible battery usage...this went from day 1 to about day 5....then Stage 2 started around Day 5 where battery life improved for 80% of the situations, but strangely sometimes something like checking an email would suddenly cause battery usage to plummet, but then it would become ok again for the next few hours. I could never really predict what would cause battery life to plummet again, sometimes it would be browsing the web, other times looking at Maps, email, etc.
Stupidly, I decided to "restore" and do a fresh install of 11.4 again....this led to another whole 5 days of terrible battery life, it was during the pits of that desperation that I decided to install Beta4 and am very glad I did. I've noticed no more "episodes" of literally being able to watch battery life tick down percentage points for each email I've read.
But I did speculate that perhaps it was something specific to my (our) installation that was leading to this...only way to check would be to restore to 11.4 stock install, but not to restore any apps / config settings. Treat the phone as a complete new phone and leave off all 3rd party apps and measure battery life.