years ago the US military conducted many tests on making reliable connections. Tinning the wires is bad for 2 reasons.
The tin will cold flow as previously stated. The tinning ends at some point, making a transition from stiff to flexible.
You need to replace the connector or the entire fuse box. Then the burned wires need replaced.
In each cavity of the connector, put in a dab of dielectric silicone.
Correctly strip the wires, do not nick strands.
Put in connector and tighten screws. The dielectric silicone will exclude oxygen, and prevent corrosion.
Check them every so often, they will come loose. I suggest after first month, then every election year.
I bet some connections on your home breaker box are loose too. I tighten mine a few years back during an epic power outage. About 10% were loose.