Under what conditions can an iOS app exceed its memory footprint limit and continue running?

I would like to understand the relationship between an iOS app's memory footprint and the memory limit used by os_proc_available_memory().

Device: iPhone 15 Pro Max (iPhone16,2).
The reported OS is iOS 18.5 (22F76), but the accompanying diagnostic metadata lists iOS 17.5.1 (21F90). The OS version for this incident is not yet confirmed.

This was a very rare occurrence. We do not have reliable reproduction steps.

In the first 26 recorded samples, footprint + os_proc_available_memory() is approximately 6,442,450,944 bytes (6 GiB). In the final four samples, os_proc_available_memory() is 0, but footprint continues to increase.

From the first zero-available-memory sample to the last sample:

• Elapsed time: 2.928 seconds.
• Footprint: 6,510,375,920 to 7,245,821,920 bytes.
• Additional footprint: 735,446,000 bytes (735.446 MB).

Questions:

1. Why could the app continue allocating memory after its footprint exceeded the apparent memory limit, with footprint continuing to increase for the interval shown above?
2. The os_proc_available_memory() documentation says the function returns 0 if the process has already exceeded its memory limit. Under what circumstances can a process exceed that limit and still continue running and calling this function?
3. The documentation also says memory limits can change during the app life cycle and do not necessarily correspond to the physical memory available on the device. What specific conditions cause this limit to change?

Documentation: https://developer.apple.com/documentation/os/os_proc_available_memory

All 30 samples are below. Timestamps are Unix milliseconds; both memory columns are in bytes. Commas within numbers are thousands separators.

timestamp_ms      | footprint (bytes) | os_proc_available_memory (bytes)
1,793,468,915,891 | 2,107,497,360 | 4,334,953,584
1,793,468,916,891 | 2,035,637,136 | 4,406,813,808
1,793,468,917,891 | 1,931,844,544 | 4,510,524,480
1,793,468,918,891 | 1,311,349,696 | 5,131,101,248
1,793,468,919,891 | 1,310,268,352 | 5,132,182,592
1,793,468,920,891 | 1,297,144,768 | 5,145,306,176
1,793,468,921,891 | 1,315,003,328 | 5,127,447,616
1,793,468,922,891 | 1,330,895,856 | 5,111,489,552
1,793,468,923,891 | 1,783,930,080 | 4,658,504,480
1,793,468,924,891 | 2,133,515,728 | 4,308,836,912
1,793,468,925,891 | 2,441,863,040 | 4,000,571,520
1,793,468,926,891 | 2,710,790,352 | 3,731,611,440
1,793,468,927,890 | 3,127,452,336 | 3,314,933,072
1,793,468,928,890 | 3,348,341,760 | 3,093,961,728
1,793,468,929,891 | 3,573,834,992 | 2,868,615,952
1,793,468,930,891 | 3,918,456,384 | 2,523,879,872
1,793,468,931,891 | 4,149,520,272 | 2,292,815,984
1,793,468,932,891 | 4,449,937,680 | 1,992,513,264
1,793,468,933,891 | 4,729,449,104 | 1,713,001,840
1,793,468,934,891 | 4,902,857,600 | 1,539,478,656
1,793,468,935,891 | 5,146,340,608 | 1,295,733,504
1,793,468,936,891 | 5,348,453,920 | 1,093,997,024
1,793,468,937,891 | 5,625,376,624 | 816,976,016
1,793,468,938,891 | 5,820,412,000 | 621,580,192
1,793,468,939,891 | 6,099,595,696 | 342,855,248
1,793,468,940,891 | 6,284,112,592 | 158,272,816
1,793,468,941,963 | 6,510,375,920 | 0
1,793,468,942,890 | 6,765,426,032 | 0
1,793,468,943,891 | 7,088,387,872 | 0
1,793,468,944,891 | 7,245,821,920 | 0