Thank you very much for the additional information.
According to your screenshot, your template node already has three labels. So, you do not get that third label due to mapping your text.
In the GraphML file, all the text is in the first and second labels. It is the third label that is empty and thus "invisible".
Unfortunately, there is no easy way to select all empty labels in yEd. Usually, the best you can do is selecting one non-empty label (if all your labels are inside nodes, use CTRL+click to select the label), expanding the selection to all labels using CTRL+A, manually removing all non-empty labels from the selection using SHIFT+click, and finally deleting the still selected labels using DELETE. (Yes, it is tedious.)
However, in your case it is actually easier: Fix your template node and reapply your property mapper configuration. (The easiest way to do that is probably creating a new node, adding only one additional label, and finally adding said node to your user defined palette section.)