FreeBSD 15.0 nfsd makes all write accesses EPERM

Our CI uses this script to quickly setup a FreeBSD NFSv4.1 server for
testing. The script worked with FreeBSD 14.2, but with FreeBSD 15.0
all files are suddenly read-only (write access gets EPERM):

sysrc rpcbind_enable="YES"
sysrc mountd_enable="YES"
sysrc nfs_server_enable=YES
sysrc nfsv4_server_enable=YES
sysrc nfsuserd_flags="-domain GLOBAL.LOC"
sysrc nfsuserd_enable=YES
sysrc nfscbd_enable=YES
printf 'vfs.nfs.enable_uidtostring=0\n' >>'/etc/sysctl.conf'
printf 'vfs.nfsd.enable_stringtouid=0\n' >>'/etc/sysctl.conf'
printf 'vfs.nfsd.issue_delegations=1\n' >>'/etc/sysctl.conf'
mkdir /nfsdata
chmod a+rwxt /nfsdata
printf 'V4: /\n' >'/etc/exports'
printf '/nfsdata -network=80.247.17.0 -mask=255.255.255.0  -sec=sys\n'
>>'/etc/exports'
service nfsd start


The failed NFS4.2 op looks like this in wireshark:

Network File System, Ops(8): SEQUENCE, PUTFH, SAVEFH, OPEN, GETFH,
GETATTR, RESTOREFH, GETATTR
    [Program Version: 4]
    [V4 Procedure: COMPOUND (1)]
    Tag: open
    minorversion: 2
    Operations (count: 8): SEQUENCE, PUTFH, SAVEFH, OPEN, GETFH,
GETATTR, RESTOREFH, GETATTR
        Opcode: SEQUENCE (53)
        Opcode: PUTFH (22)
        Opcode: SAVEFH (32)
        Opcode: OPEN (18)
            seqid: 0x00000000
            share_access: OPEN4_SHARE_ACCESS_WRITE (2)
            share_deny: OPEN4_SHARE_DENY_NONE (0)
            clientid: 0x0000000000000000
            owner: <DATA>
            Open Type: OPEN4_CREATE (1)
                Create Mode: UNCHECKED4 (0)
                Attr mask[0]: 0x02000010 (Size, Hidden)
                    reqd_attr: Size (4)
                        size: 0
                    reco_attr: Hidden (25)
                        fattr4_hidden: No
                Attr mask[1]: 0x00004002 (Mode, System)
                    reco_attr: Mode (33)
                        mode: 0644, Name: Unknown, Read permission for
owner, Write permission for owner, Read permission for group, Read
permission for others
                    reco_attr: System (46)
                        fattr4_system: No
            Claim Type: CLAIM_NULL (0)
                Name: u
                    length: 1
                    contents: u
                    fill bytes: opaque data
        Opcode: GETFH (10)
        Opcode: GETATTR (9)
        Opcode: RESTOREFH (31)
        Opcode: GETATTR (9)
    [Main Opcode: OPEN (18)]

Suggestions, please!
(we're aware that this setup is stupid and insecure, it is only
intended for a CI behind a firewall)

Aurélien
-- 
Aurélien Couderc <aurelien.couderc2002@gmail.com>
Big Data/Data mining expert, chess enthusiast