I tried wg syncconf wg0 <(wg-quick strip wg0) without success.

Only restarting wireguard by systemctl restart wg-quick@wg0 makes it
respond with a Handshake Response packet.

This happens really rarely and especially on OpenWrt 19.07 as a client.

Other clients can handshake properly while the server fails to
complete the handshake for the OpenWrt device.

Packet Captures on boths sides are in the attachments.

Server:
5.9.0-0.bpo.5-amd64 #1 SMP Debian 5.9.15-1~bpo10+1 (2020-12-31) x86_64 GNU/Linux
wireguard-tools v1.0.20210223 - https://git.zx2c4.com/wireguard-tools/
wireguard-dkms/buster-backports,now 1.0.20210219-1~bpo10+1

Client:
4.14.221 #0 SMP Mon Feb 15 15:22:37 2021 armv7l GNU/Linux
wireguard-tools v1.0.20191226 - https://git.zx2c4.com/wireguard-tools/
kmod-wireguard 4.14.221+1.0.20200611-2

Arınç Ü.