I am on joomla 4.2.5, JCE 2.9.32, Safari 16.1, php 7.4.26
In the existing html text below, I have replaced the semicolon with colon so the  : will not disappear.
<p>Hello, :How :are :you?</p>
If this text is already in an article, and I have "Keep non-breaking spaces" set to YES, then when I save the text, it is returned to the database table as it existed before, that is, in the actual character sequence of " :" .
If I change my global setting of "Keep non-breaking spaces" to NO, open the above article, and save it, the " :" disappear but in the database table for the article, instead of being saved as a regular space hex 20, they are saved as two hex bytes "c2 a0" .
If I display this article with Safari, and use the Safari Web Inspector to copy the paragraph, it is obvious that the  : are still there.
I would hope that JCE would leave them in the  : form so existing  : could be searched and replaced manually.
Please see two attached hex dumps of introtext field of article in concern. Look for the word "Hello". One is result of save of existing article with global setting to "Yes", and the other is result after the save of existing article with global setting to "No".