Here it is

looks like mechanical damage ... meaning a boo boo ...

but i cant tell from the pic.. if there is actually a brown center on the leaf ...

i would probably pull back the sphagnum [or whatever] top layer of mulch/media .. and see what that leaf looks like where it attaches to the plant .. and maybe just pull it off... out of sight.. out of mind ...

ken

Just to the lower right of the browning leaf is a sterile/nest leaf, that's the way they normally grow. The browning leaf does look a bit like a nest leaf but they usually stay green all over while they're spreading over the base. Later they do brown off.

It's not natural for these ferns to grow forced into pots. They naturally grow on tree trunks/branches or rock faces. Drainage is very good, something which becomes a bit more difficult in a pot. It's easy for rot to set in, you need to be careful with watering.

Watch this leaf carefully, it could still grow out okay.