I receive about 80 channels from my cableTV-provider, but numbered between 1 and 450. A very long list with lost of empty slots.

So I decided to create the list of Favorite Channels and made my list of approx. 40.
After storing and testing everything seems fine. TV off, TV on. No problem.

However after two days the TV only has 13 channels left in the favorite list. The other just disappeared in the favorite list, but are still marked as favorite in the general list. The TV does not count any more after 13 in the list of availables.
How can I solve this?

I have a 40PFL5507.

Thanks for support.