I've just found out that - when using post pagination with
<!--nextpage-->, RSS feeds will no longer show the whole post and a hint that there are more pages is missing.
I tried to insert an if-clause into the single.php within the loop to see if it is a feed (via "is_feed()"). I accessed the number of pages via "global $numpages". If this variable is greater than one, a message should appear in the RSS feed that there is more than one page, leading to "....." . the_permalink() . "/2/" for the second page.
It doesn't work. The message does not show up in the feed.
If the feed readers think that they get the whole feed, they are actually missing out on the other pages.
Thanks in Advance!