Maybe this was just that night or just this one restaurant, but it kinda shocked me.
When I moved to the US, I was a bit confused at first that people did not follow the same table manners even when they were using fork and knife. Sometimes they cut food first and then only used the fork. Often they would leave the other arm below the table, which I have been taught not to. It is perfectly normal to not to finish your plate in the US.
I've got over it, and do the local way now. I do not notice whether I or someone else leave their arm hanging. I especially enjoy the ability to not finish my plate and take leftovers home instead (which is possible in Germany, but the instinct of finishing, sometimes forcibly, your plate made it less common). Customs are just customs.
But leaving food on the plate wasn't acceptable in my house not because my father was German, but from my mother, too, because they grew up (or parents grew up) in a time when there often wasn't enough to go around and wasting food was a ridiculous and rude indulgence.
About the knife and fork thing, I believe that might partially be the difference between continental and Anglo-American style of handling the fork. [1]
[1] https://theetiquetteconsultant.com/blog/2018/1/26/styles-of-...
This may be a "German thing"; I remember being a bit surprised when I ate lunch with a vendor from Germany and he ate a hamburger and french fries with a knife and fork.