glabels garbles output address, V 3.4.0, Ubuntu 18.04

I have a template set up for Avery 5262 labels in glabels. Address text is read from an external text file and merged. I normally select one or more addresses, mark the label to begin printing at, and click the print button. Has worked like a charm for the last several years.

Recently glabels has started printing part of the address as garbage, inserting spaces and random characters. I have checked the address file with a hex editor and there are only ASCII characters present, exactly as entered. The problem occurs whether I print directly to the printer, or print to file (PDF).

For example, here's a line as entered in the merge file:
  Jane Doe: 543 Main St: Anytown, VT 05455:

See the attachment for what's actually printed.

This bug might be related to 1799867, but I don't know.