Found a workaround

var redisConnectionString = builder.Configuration.GetConnectionString("redis-cache")!;
builder.Services.AddSignalR()
    .AddStackExchangeRedis(redisConnectionString, options =>
    {
        //options.Configuration.
        // Optional: set configuration options
        options.Configuration.ChannelPrefix = RedisChannel.Literal("MyApp");
        options.Configuration.AbortOnConnectFail = false;
    });

Also opened an issue at https://github.com/dotnet/aspire/issues/10211