All blogging tips recommend to have the full RSS feeds. To my unpleasant surprise, I found that my feeds are partial, disrespecting the option "Full text" is set. The explanation is simple.
The comment next to the option says:
Note: If you use the <!--more--> feature, it will cut off posts in RSS feeds.
I think WordPress didn't have this misfeature initially, but it was introduced during upgrades.
Fortunately, I've found a solution:
Full Text Feed plugin by Ronald Heft, Jr.