It will depend on your iOS version, Tom.
Until iOS 7 the Camera Roll and My Photo Stream were strictly separate and it has been easy to understand what has been happnening.
In iOS 8 Apple did not show My Photo Stream at all as a separate album any longer, but My Photo Stream has been included in All Photos. We could no longer import selectively from My Photo Stream or manage it. Many users complained, and since iOS 9 we can see again a My Photo Stream album on iOS devices, but it is a strange mongrel. The My Photo Stream album seems to be linked to the Camera Roll. When I tested on iOS 9, ,0 and 11, photos that I deleted from the Camera Roll on my iPhone, vanished from my Photo Stream as well, and even from the Camera Roll on my iPad, at least when I did delete them while the 30 days were not yet over. Several users reported a similar problem in here forums.
I have not tested My Photo Stream on iOS 12 at all. Perhaps this problem has been fixed in the most recent iOS versions. But since Apple is about to deprecate My Photo Stream and makes it no longer available for new AppleIDs, I doubt, that much bug fixing has been done in the current iOS version. While I used My Photo Stream I found it the safest strategy, to leave streamed photos alone, as long the 30 days in iCloud were not yet up.
