It's definitely not from the strap, because the same thing happens with my Fluke that doesn't have a strap. I think it's a combination of the nylon string and the shorter string length of a ukulele. It ends up with the sound created by plucking the string being a larger percentage of the overall sound made by the string than with a guitar. It doesn't bother me much, because it's not as if I'm recording an album with it.
If I use the Mel9 brass sound and turn down the attack speed on the box, the brass sound will come in a second after the uke sound. So I did that, and then ran the whole uke and effect output through the compression/distortion/delay effect I like to use, from the effects processor module in my Yamaha MOX6 synthesizer. I really liked the sound it made, so I needed something to play with it. I looked at what printed chord sheets I had handy that I hadn't recorded yet. And what I had was -- "Against All Odds" by Phil Collins. I had printed it out to try with the Mel9's string section sound. It made no sense at all to use it with this particular sound. Until I jammed on it for a while, and then it made perfect sense.