Continuous Label Length not saved in File

[GralfR]

I use glables v3.99-564 on Linux Mint 20 to print continuous labels on my DYMO 450 Duo.
Because of missing templates for continuous labels I created my own templates (example).
When I save a design to a file and open that file again, the design looks good, but the property length is zero and locked. So changing the length of saved continuous labels is not possible.

Only workaround: Change the poduct type of the opened file. This unlocks the length property but also resets the length back to the default length of the template.