Headers / Frontmatter limitations with large amount of list field items?

Hey all,

i have a page with a custom blueprint including a list field.
That list should contain ~500 items at the end. Unfortunaly after 250 items its not longer possible to add more items via the backend.
I still can add items manualy to the header in the .md-file, but as soon as i save the page again in the backend - its gone.

I dont have any erros in the grav log.
Maxing memory_limit and post_max_size didnt help.

Anyone an idea what could cause that behaviour and how to solve it?