Suddenly Siri started to drain battery on iPhone SE (iOS 12.3.1)
Non-solutions:


turning Siri Suggestions off
turning Siri off "completely"
playing a song in iTunes/Music (suggested here by DRH255)


These made no change.

Solution:


disable the option Settings / General / Keyboard / Enable Dictation (got the hint here and in the above link)


Funny thing, I did not have that option at all, because it appears only when a keyboard language that supports it is added. So I had to add "English (United Kingdom)" to my keyboards, then the dictation option appeared, I turned it off and... the problem is gone.

PS: This post on reddit suggests it is not really draining the battery, only the indicator is showing the wrong value....

*edit*: this seems to (also) be the case. I left my iPhone to drain the battery completely and when it reached 1% it kept going for 5 more hours, always showing 1%. Before that it decreased by 2-3%each hour.

PPS: It still uses about 15% during night, which is too much according to some, but that is probably a separate problem.