You need to add the [QSA]
flag (“query string append”)
RewriteRule ^apps/([A-Za-z0-9-_]+)/?$ index.php&app=$1 [L,QSA]
For page 301 redirects with the [R]
flag as opposed to internal rewrites like this one, the query string is automatically appended. However, you must force it with [QSA]
for the internal rewrite.