Help calculate MTU, ISP's 1448

[STR .]

Hi,

I have Fiber to our apartment complex basement, from there Cat6 runs to
each apartment. The ISP/apartment service provider suggests an MTU of
1448, which I set for the PPPoE interface on my OpenWRT router.

I read 
https://lists.zx2c4.com/pipermail/wireguard/2017-December/002201.html
which comes to (assuming 1500 byte MTU) to 60 bytes (IPv6) to 80 bytes less to account for Wireguard protocol overhead.

Using this info, I tried an MTU of both (1448-80=1368) and (1448-
60=1388).
As my ISP assigns only IPv4, I expected an MTU of 1388 to work, which I
set on the Wireguard interface in OpenWRT.

However, when set to 1388, almost everything works except any Google
related sites like Maps, Gmail, YT etc.
When set to 1368, everything works and it's the way I have it setup
right now.

What am I missing here?
Why won't Google sites load via my WG VPN when the MTU is set to 1388?

If it helps, I host the WG server on Google's cloud platform and was
informed that GCP has an MTU of 1460 bytes.

Thank you,
S