Is there anybody who can answer my question please...? Or confirm that my problem is real..?
That is the way OL's in HTML work. You'll need to crack open the EPUB to fix it.
I have spend a lot of time on trials and errors using the CSS you advised in another answer, and I agree that this is the best way to do it.
It works perfect and can be copied to every other iBook I have to create in this series.
I will create a workaround for this problem and go on with the business.
If you're doing a series, an even better way is to get the CSS the way you want, then in the EPUB export, select your massaged CSS file instead of editing InDesign's.
That's what I ment by 'copied to every other iBook I have to create in this series' ( I mean 'linked in ePub export pop up').
I use a small CSS file containing what I need and have no surprises anymore. I learned a lot this weeks...
And concerning InDesign (as I always say in the IT-world): For every solution there is a problem...