Looks like a classic text pre-pass issue. There are 2 ways of fixing it (and probably a couple ways to hack around it). If you know the size of the parent container (the vertical box hosting text sits in a canvas panel, so that’d be the size of the created slot):
Set Wrap Text At to the desired value. Do account for padding if there’s any. This should work well if the container size does not change.
If the container size may change run-time and/or you add/remove content dynamically (especially User Widgets choke full of text), consider calling Force Layout Prepass early.