I have narrowed the problem down a bit.
Problem does not happen on iPhone 8 with iOS 16.5.1
Problem does not happen on iPad Pro with iOS 16.2
Problem DOES happen on iPhone XR with iOS 17.2.1
So it appears the problem happens on the newer version of iOS but not on older versions.
Does anybody know what changed, as far as SSL goes, in the newer versions of iOS - and if so, what can I do to fix this? Can something be done on the client (iPhone) end or do I need to make changes on my API server? My API server is not using the lastest SSL configurations.
Edit - I updated iOS on the iPad to iOS 17.3 and the problem now also happens on the iPad, whereas before with iOS 16.2 it did not. So the problem is definitely related to the iOS version installed.