I've never heard of OP's convention. But if I had to guess, it's this:

• It's slow to input text on an onscreen keyboard compared to a physical one.

• Mobile vendors try to reduce the number of keystrokes via predictive text and other tweaks in their onscreen keyboard software.

• One common optimization (which I do not like and have off) is to try to reduce the effort to terminate a a sentence.

• On iOS's keyboard, tapping space twice inserts a period, then space. This is an easy action to perform.

• I would assume that many iOS users are thus trained to only terminate sentences this way, and not to explicitly use periods. A trailing period requires extra effort and an unusual keystroke.

• As a result, iOS users tend not to put in the extra effort, and so their sentences tend not to have a trailing period if not followed by a subsequent sentence.

• For these users, the norm then becomes to omit a period on the final sentence, and so explicitly adding it looks like the user has gone out of their way to specially add punctuation. The trailing period then acquires semantic value, meaning.

I expect that the whole thing stemmed from some random engineer at Apple just banging away trying to get average typing speed up, not spending a lot of time thinking about any linguistic or social impact.

It could also be that Microsoft or Google do that by default

but I don't use their default onscreen keyboards, and the descriptions I can find online of their default behavior sounds like they don't.