Cap can insulation?

[Helmholtz]

No surprise .
That's why I asked whch cap you were talking about.
Schematic tells you which cap can could have high voltage wrt ground.

[g1]

Did you check the replacement caps to see if the can is connected or not? I've had some where there's no connection between the terminals and case.

The LCR caps in Marshall's were like that. I guess they are out of business and some are using ARS caps as replacements for them:
https://www.arselectronics.com/ARSWE...age2.html#dual

[SoulFetish]

Dual Wall heat shrink.