No, Roku is not doing anything within a 3rd party app/channel. If Rumble is having problems, it's something that Rumble has to fix. It might be a server problem on their end, or they have something programmed wrong in their app. There's no way Roku can block content within a channel provided by another service.
Roku doesn't "censor". If a channel is removed from availability, it means the provider has violated the terms of service. The same rules apply to all channel providers. If you consider not allowing certain adult content on their devices, I guess you could call that censorship. But remember that the first amendment only restricts the government from hindering free speech. Private companies are will within their rights to not allow certain content within their sphere of control. Newspapers are not required to publish everything that is sent to them. TV news stations the same. Online providers have the same authority.