I had a portal with 2 VRMH that was burnt out. It was attacked and destroyed. I installed 8 new resonators, added a VRMH, and still the portal is burnt out.
Moto G stylus 2021 Metro variant.
Ingress prime 2.74.1
Burnout is only reset with a Heatsink, not a Multi-Hack. Multi-Hacks only give you more hacks until the burnout, but once the portal is burned out, you need to use a Heatsink.