Hello Matthias,
The problem, and this is almost always the case, is that the columns are explicitly typed as numbers with the Excel worksheet. The engine that we use on Windows to read the files actually does a good job of showing us the types, which in this case is to the detriment of the desired outcome. When the type is not General, we strictly honor the typing. When we convert the numeric, of course it comes out missing. This behavior has been requested by users over the years (“just give me what I asked for if I went to the trouble of typing it”), but it has its downsides.
The solution for you in this case is to type the columns as Text or General and then import it. I’ve attached an example where I typed it as Text, but again General will work.
I hope this helps,
Brian Corcoran
JMP Development