I tested two Linux WireGuard peers, and they don't suffer from this MTU
"overflow" problem. I believe a bug was introduced by @cjeker in commit
c061998[^1]:

- plaintext_len = min(WG_PKT_WITH_PADDING(m->m_pkthdr.len), t->t_mtu);
+ plaintext_len = WG_PKT_WITH_PADDING(m->m_pkthdr.len);

Linux, FreeBSD, and older OpenBSD versions clamp the plaintext padding
so that the MTU is not exceeded.

[^1]: https://github.com/openbsd/src/commit/c06199859734d958552a581d72b4c0f910e68d7c