Please link to the diffs for the actual edits where you are seeing this issue.

This is the diff where I added the template and the category:

https://pl.wikipedia.org/w/index.php?title=React.js&diff=prev&oldid=69755668&diffmode=source

I added this because React.js was in the template even though it's a library.

Here is another example there was missing [thumb| that I've added but it turns out that it was there the diff say that I've removed |thum| and added |mały| even though the Wiki page was broken and the image was full screen:

https://pl.wikipedia.org/w/index.php?title=Pol%E2%80%99and%E2%80%99Rock_Festival&diff=prev&oldid=69789380&diffmode=source

The problem may be because MediaWiki sends an email with a notification when someone changes the page. Maybe this is the real issue of why I don't see the latest changes and delete something by accident.

It happen again, didn't get a warning about the conflict I've changed duplicated Wikilink same name was in the link, and the alias

[[Numb/Encore|Numb/Encore]] => [[Numb/Encore]]

https://pl.wikipedia.org/w/index.php?title=Linkin_Park&diff=prev&oldid=70131065&diffmode=source

I think that I've opened the link from the email the same as before. It was a trivial change that's why I didn't check the preview. But since the email was sent there were probably new edits to the Article.
Maybe the history page should give a warning that you see an outdated page and there are new edits. I think this is the real problem.

This is just my hypothesis, but I would verify if it's true.

This happens again, I'm 100% sure that this happens when you open the history page.
I followed the edit by the newcomer and edited the page to correct his changes, but later there were more changes and MediaWiki didn't warn me about more changes. You can see that I've added a description that I've added an image:

https://pl.wikipedia.org/w/index.php?title=Stanis%C5%82aw_Andrzej_Moskal&diff=prev&oldid=70469840

But I've added an image to Infobox and added back a grave image. But only the description was added because someone else added the same change but without an image description in Infobox.

Since you were using the 2017 wikitext editor for all of these changes, I am guessing that this is the same problem as T337071 – that is, the warning message you want already exists, but it's not easy to notice:

I'll merge this task into that.