Does the Transalp have a USB-outlet or do you mean a USB-charging-adapter plugged into the 12V outlet. Many of those (cheap) USB-chargers do not deliver the current ratings they claim.
Another issue I had in the past, was that the charging adapter didn’t fit tight enough and lost contact from time to time.
This one is working for me
https://www.louis.de/artikel/usb-zigarettenanz-stecker-doppel-usb-steckadapter/10032696?list=a749c2f568f7a16367478c34346e7b87&filter_article_number=10032696