The Transit Mini Supports Hot Failover (1 Link at a time) and WAN Smoothing (Same Traffic down all Links). The Spec sheet does not list WAN Bonding (2 Links Aggregated), so I think that’s why you are seeing what you are seeing.
If you do the Weighted Balance option under outbound policy, there will be some applications like Netflix and Prime Video that use 4 individual data streams, so they will give you an aggregated performance, but it won’t be true for every application.