I just posted a video is the exact same thing. Didn't notice it live but when i looked at the video i saw it. Seems to happen right around 60 seconds after i turn on. Super expensive amp with build date of sep 2021 and bought in 2022. The best part is about 3 months after i bought the Blues Jr, i bought a Katana 50 and hardly ever used the Fender. Junk!
WRT 1k screen resistors are a good idea, they will help a little. It's worth measuring the screen voltage (drop through 1k R) at max power to verify what the change actually is. A good option is EL84M / 7189A if owner can absorb cost. Mil spec, higher plate voltage rating and power dissipation EL84.
