Here’s the fix: in
$blogroot/wp-includes/post-template.php, edit line 453 (for me) to read:
if ( preg_match('///', $content, $matches) && !is_feed() )The bolded part is added to ensure that the more tag shenanigans don’t take place in feeds. Many thanks to Manbou Haven for the answer.
Anyway, this should put an end to my RSS feed issues. If anyone is noticing lingering weirdness, do let me know.