Hi There,
I had the same issue with my new 2019 TC Limited (I'm in the UK). The dealer had my vehicle back 3 times. After giving the dealer a detailed explanation of the problem and a possible suggestion, the workshop couldn't find a problem! When I collected the vehicle I asked what tests did they carry out. It turned out the technician tested the inverter with an iPhone charger and for only 10mins. The second dealer visit I left in the van for testing a desk lamp with an LED bulb and clear instructions to keep testing beyond 15 minutes. The third visit to remedy the issue was to reload the vehicle software. Since then all has been working just fine.