Halide shouldn't be afflicted by the lost photo issue you're talking about. Normally, if you close a lock-screen extension before a photo is written, it won't be in your library. This can be a problem when the user quickly smashes the capture button and your photo queue gets backed up.

Luckily, the unlocked app gets a chance later to access the content of lock-screen capture sessions. So our strategy in Halide is to write the intermediate files to disk as soon as you initiate a capture; this should take a split second. When we have uninterruptible time, later, we make sure the photo finished writing to your library, and if not, we try again.

However, Apple's documentation isn't clear if the OS will "wake-up" your main app to process unfinished photos, or if it relies on the user launching the app. I'll just say that I dog-food Halide every day by capturing photos of my toddler, and I can't remember the last time I lost a shot.