Rectifier tubes flashing

I put two NOS TungSol 5U4GB rectifier tubes in my Cary SLI-50 integrated amplifier. They have great, truly vintage sound, but when I switch my amp from the standby mode into the operating mode, they flashe very bright for a second and then dull down to the normal soft orange glow. It does not affect the sound.

Is it a problem or it is just the characteristic trait of these tubes?

What is interesting, no such flashing occurs when I insert these rectifiers in Cary SLI-80 integrated amp which does not have a standby mode.

I would appreciate any explanation on this matter.

Best regards,

Alexander