Strange htaccess behavior

I set up Multisite and it works on the primary site. I have a sub folder site, IAMC at /iamc/ This site works too. But it has this strange behavior.

subfolders off IAMC on the first level throw 404 errors
those beneath, however, do not.

I get no errors when disabling permalinks.

See it in action:

Works: https://www.globalchange.umd.edu/iamc/
Does not work: https://www.globalchange.umd.edu/iamc/annual-meetings/
Works: https://www.globalchange.umd.edu/iamc/annual-meetings/testing/

My htaccess:

# BEGIN WordPress
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ - [L]

# uploaded files
RewriteRule ^([_0-9a-zA-Z-]+/)?files/(.+) wp-includes/ms-files.php?file=$2 [L]

# add a trailing slash to /wp-admin
RewriteRule ^([_0-9a-zA-Z-]+/)?wp-admin$ $1wp-admin/ [R=301,L]

RewriteCond %{REQUEST_FILENAME} -f [OR]
RewriteCond %{REQUEST_FILENAME} -d
RewriteRule ^ - [L]
RewriteRule  ^[_0-9a-zA-Z-]+/(wp-(content|admin|includes).*) $1 [L]
RewriteRule  ^[_0-9a-zA-Z-]+/(.*\.php)$ $1 [L]
RewriteRule . index.php [L]
# END WordPress

I’m at a loss. Help?