At which point it stops truncating the string and sends the full text all the way through the system. Ah-hah, I think, this must be a heisenbug (adding debugging code can have very subtle effects on program behaviour). I'll remove my debug node from the flowchart, rejoin the original pair of nodes together, and upload the chart.
And it's still working. Ok, I think, the act of converting the chart for the unit must have changed it slightly from the original. It shouldn't do so as that implies non-deterministic code somewhere, but that'd at least explain what's going on. So I dug out the original already-converted chart and transfer that, thinking that this'll now restore the broken behaviour (when debugging it's always helpful to be able to reproduce the original bug - if you can reliably reproduce it, you can use a debugger to see every step involved and often quickly spot the actual problem).
Except it's still working!