This problem is since version 1.21, there is the same problem on the digitakt since version 1.11. To bypass it, copying your patern to another project and copying each data on a blank patern on which you have previously defined the length. then transfer the unbuged patern to the original project.
The problem comes up regularly when you change the scale per track settings.
the bug has still not been fixed since June 2019!
I invite you all to contact the support service so that the Elektron team corrects the bug.
digitone-digitone-keys-1-21-bug-reports
digitakt-1-11-bug-reports