Bugs
Have you refreshed? Have you refreshed enough times? I find that sometimes the inventory doesn't get updated immediately and I need two refreshes to get things to show up properly; although I've only seen this happen with trades.
Answer
Fixed
Silkcom contacted Lan and found the issue (and replaced the lost cart). This will be fixed with the next restart, for now we wouldn't try making mining carts
I Made a mining cart yesterday on 2/13/2017 and it also stated that it was successful and never showed up in my inventory. This is still an active bug and not fixed. I did not know before hand that there was a issue making mining carts or i would have asked if it was fixed before i made one.
Customer support service by UserEcho
Silkcom contacted Lan and found the issue (and replaced the lost cart). This will be fixed with the next restart, for now we wouldn't try making mining carts