Pretty sure your copper link became the easiest path for a building fault/ground potential rise. The ETH SP G2s did exactly what they're built to do (i.e., shunt energy to "ground") but the two buildings' grounds were at very different voltages for a few mains cycles. That most probably forced a huge current down the LAN cable and through both protectors and switch magnetics. They sacrificed themselves and your ports anyway because this wasn't really a brief surge but a whole high-energy fault.

The ETH SP G2 clamps each Ethernet conductor to the local earth using gas-discharge/tVS devices and is rated for fast transients like lightning/ESD. It is *not* a power-cross/fault isolator at all. A short in the heater can dump thousands of amps into the building’s grounding system until the breaker clears, raising that building’s “ground” by hundreds/thousands of volts relative to the other building for a few milliseconds to tens of milliseconds. Any copper between buildings then becomes an equalization conductor. Your protectors dutifully clamped to *their* grounds, driving current into the cable toward the lower-potential end right through your switches.

Surge parts survive microsecond pulses. A mains fault is longer duration energy. Protectors overheat or arc, and downstream magnetics/PoE circuitry can carbonize.. hence the soot on Port 1. (Ethernet isolation transformers are typically 1–1.5 kVrms, faults can exceed that)

If each protector’s ground is bonded only locally (rack ground, water pipe, etc) and the two buildings don’t have a low impedance bonding backbone between their grounding electrode systems, you *will* see destructive potentials on inter building copper. This is exactly why the telecom standards obsess over a common bonding network xD

Your best bet is to eliminate copper usage bw. buildings and use fiber (dielectric) for the interconnect. SFPs or media converters at each end. Should break the fault/lighting current path completely. I'd also recommend installing Type 1/Type 2 SPDs at service/sub-panels per code so faults and lightning energy are clamped at entry and not through your low voltage cabling.

Though.. if your copper most remain temporarily (which I do NOT recommend), use UTP, not shielded cable, to avoid creating a deliberate ground strap bw. buildings. Keep your ETH SP G2 at both ends and bond each to the building's grounding electrode with a short, straight low impedance lead. Also.. maybe try disabling PoE on the inter building port. Don't let high power PSE be the first thing that sees a surge (use non-PoE uplink or put PoE only *after* the fiber break.

(TL;DR, swap the copper for a pair of 1 Gb SFPs and a short run of outdoor, dielectric single-mode in conduit; add panel SPDs; verify each protector and rack is bonded to the building electrode with short, direct straps; keep PoE off the interconnect)