interfaces_ppps_edit.php: L2TP and PPTP WAN-type interface editing has broken input validation
On interfaces_ppps_edit.php when an L2TP or PPTP interface is created or edited, it cannot be saved.
Both types have an expanding list of "IP/Gateway" inputs for each selected interface, but input validation is not checking the arrays properly and it is rejecting any input. When the input validation is removed, the values are not stored in the config either.
Going the other way seems to work, starting at interfaces.php and then making an L2TP interface there, it works. If you edit that working entry at interfaces_ppps_edit.php it shows the values entered on the other page, but on save they are lost, as above.