I'm a bit confused about this. Do people turn off auto capitalisation on their phones? I very rarely have to press shift on my phone
Using the chat/IM style outside of that context just doesn't work and looks really odd, like it's obviously someone who didn't learn those norms and is now mimicking them without understanding them.
Or another example: "Call me" is a just a regular "let's chat about something", but "Call me." is "something bad happened I need to tell you about, so prepare yourself".
Interestingly, you're actually partially doing what I described on 2 of your 3 messages in this chain - you left out the last period because HN formatting makes it obvious where the sentence ends. So even if this norm did apply here (it doesn't really), you're not using the serious tone of voice.