iOS 11.3.1, When does iPhone Camera app choose JPEG format when "High Efficiency" is chosen?

Hi Apple,

FileMaker developer here. May I ask when does iOS 11.3.1 choose JPEG format vs HEIF format when in the settings "High Efficiency" is chosen?

A user hasn't touched the settings for the camera (photo format) and it is set to "High Efficiency" but I found out most of the pictures taken are in JPEG format. Somehow today there are 3 pictures taken in HEIF format (or compressing using HEIF). It creates a problem in the custom app I built for the user because HEIF isn't viewable in some browsers. I would have assumed selecting "High Efficiency" in the setting should mean ALL photos are saved in HEIF format but apparently that isn't the case. The bigger question for me is that if I want to enforce JPEG format, I choose "Most Compatible" right? but does that guarantee me that ALL photos taken are in JPEG format? Would it suddenly give me some pictures in HEIF format? I am not accusing but perhaps there is a bug there or I am not understanding something correctly. Below I have attached 2 of the many photos taken today by the user and one of them you can see is in HEIF while the other is JPEG. There were about 25+ photos taken and 3 of them appear to be HEIF and all others are in JPEG.

Cheers. Love Apple products.

https://www.dropbox.com/s/8dw7kdpkw2ebyr9/HEIF%20vs%20JPEG%201.jpg?dl=1

https://www.dropbox.com/s/hnvgz8opqfr0om7/HEIF%20vs%20JPEG%202.jpg?dl=1

regards,

K