Menu Items disappearing/being empty on save or post edit

i got the same problems several times ago, and it’s caused by file permission in linux.

for make sure about the problems, would you like to show us about the errors by adding below code in your wp-config.php

define( 'WP_DEBUG', true );
define( 'WP_DEBUG_LOG', true );
define( 'WP_DEBUG_DISPLAY', true );

be better if you give web console message to when you clicked the save button.