Custom post type fields values randomly disappear

I have set up a WordPress website that uses multiple custom post types to use different types of content, such as portfolio-items and team-members. They have extra fields, such as ‘name of the client’ and ‘responsibility in the team’ and rely on ‘featured images’.

Now, when entering or changing these posts, values seem to randomly disappear. For example, when changing someone’s responsibility in the team, upon saving the post the featured image will be gone (or the other way around).

I have not yet been able to reproduce the bug, but it seems to happen again every once in a while.

What would be the most likely cause for such a problem? Can it lie in the order of my code in functions.php (to create and save the custom posts/fields)?

Any help highly appreciated.