Double check for an applied character style, and check for local formatting overrides (you can remove them by holding down the Alt/Opt key and clicking on the style name). Another trick for removing overrides is to use find/change to search for a style and replace it with itself.
Reapply the Paragraph Style with holding the ALT key, or in the Paragraph Panel's menu "Clear Overrides".
You should ALWAYS use styles when using text, so you should not have even one letter with "no paragraph style" in use.
Character Styles can also be used via GREP and Nested Styles and Bullets and Numbers inside a Paragraph Style.
(I don't want to remove the clues I already typed, but see the new information bolded below).
As I mentioned previously, there are no underlying paragraph or character styles. To make this perfectly clear, I have deleted the text, removed all character and paragraph styles, and retyped the text. Just to make sure it wasn't something with the text frame, I created a new text frame and repeated the experiment. Each time, manually typing the text, applying the paragraph styles, and then deleting the carriage return between body and heading.
The problem lies somewhere with the style definitions. (no it doesn't)
Oh, here is something new. I created a new blank document. Copy/pasted the previous text, which took the styles with it, and the same problem occurred. The styles aren't showing anything unusual in their definitions. Basic paragraph is showing it's based on NONE with next style being the same. The heading is based on Basic with just a font change.
Hey, its a system setting, not in the document. I just opened a previous document where I know this problem did not exist, and suddenly, it exists there too. This aligns with my memory of running into this once before. There is some obscure setting that causes it, but I can't remember what it is, and I am pretty sure it is something that isn't even obvious for the symptoms.
I tried replacing the 2 preferences files and it did not correct it. I also exported the file, but because this is happening across other documents, I knew that wouldn't be the problem.
Is there any group of settings that are similar to FrameMaker's structured documents? I haven't touched Frame in a few years either, but I vaguely recall it was something along the same lines as that.
I guess I should back up and find out whether I am just confused or not. I checked with a recent client and he said his documents behave the same way. So now I don't know if this is normal and I'm just confused about it.
I could swear that prior to last evening, my system would roll heading text into the body style if I deleted the carriage return separating them. I'm 90% confident that my system used to roll the heading format into the body format if I deleted the carriage return.
Which is the normal condition? Is it possible I used to have some obscure setting from years ago that suddenly reverted back to normal?
Here it rolls the text back into the previous paragraph without changing the format and shows the original format as a local override.
OK, so that's 2 votes that I am crazy. I can accept that. But is there any chance I could have had something different on my system in the past? I'm not even sure what search terms to use to find out.
I've never installed any aftermarket add-ons, but I did do a lot of (forgotten) customizing several years ago, that may have even included going into text files to make changes.
I don't recall that it was ever different, but I'm old and losing some memory.