ironSource ad network rejects the advert due to slow loading time on iOS 15+ devices. These have a CPU lower than the Bionic 12 which debuted in the iPhone XS.
This is likely caused by a known bug in the iOS 15 release which also affected the loading of videos in Interactive Templates creatives, which causes mp4 files to reload, and therefore reducing the start-up time.
- As of this writing, the only solution is to improve the start-up time with optimization as opposed to a direct fix of this bug. One easy way to achieve this is by using a Luna shader variant file. This file tells Luna what shaders are exactly needed in the game and therefore Luna does not need to compile all different shader variants on start-up. How to create the shadervariant file can be found here: Optimising Shaders - Developer Docs
For more start-up optimization tips check this link: Measure and Optimise Startup Time - Developer Docs