Custom flags disappear

Ls,

I have two languages, 1 newly created and an existing one. For both I created custom (bigger) flags and uploaded them to the wp-content/polylang folder. I use the snippets plugin to add the pll_custom_flag function. after Languages -> settings -> URL modifications -> settings -> save, the language switcher in a Max Mega Menu displays the flags perfectly until I update a page, any page even one that has no language assigned. After publish the flags are gone and replaced by a kind of “file missing” icon and the language name.
Do the Languages -> settings -> URL modifications -> settings -> save thing again and the flags are back again.

Any ideas what happens here? Thank you in advance.