Hi,
Sometimes tinned cable tips don't hold too well from the outset in spring-loaded connectors, the rounder the worse it might be, sometimes hammering them a bit flatter improves that; tinned cables that are flexed may snap at the junction between tinned/not-tinned sooner or later or just much sooner - that might be solved with heatshrink covering half sheath/half tinned cable. You could use ferrule terminals instead of solder, much nicer all round. It's easier to place tinned in any type of connector unlike untinned where stray strands may catch/not go in and in the end it feels like threading a needle.
That looks like a reputable brand so you'd expect reliable connector, I wouldn't bother tinning it myself, just ferrules if it's something that needs to look a bit polished.