"Action was taken on a missing chunk" during three-way merges

Reproducing steps:

do a three-way merge
have a chunk that is available only on the rightmost side
in the middle panel, press Alt-Left

A backtrace appears on the console:

Traceback (most recent call last):
  File "/usr/lib/python3.7/site-packages/meld/filediff.py", line 572, in action_push_change_left
    self.replace_chunk(src, dst, self.get_action_chunk(src, dst))
  File "/usr/lib/python3.7/site-packages/meld/filediff.py", line 562, in get_action_chunk
    raise ValueError("Action was taken on a missing chunk")
ValueError: Action was taken on a missing chunk

There are bugs in the RedHat bugzilla:

https://bugzilla.redhat.com/show_bug.cgi (opened by me)
https://bugzilla.redhat.com/show_bug.cgi?id=1712715 (opened by someone else, but looks like the exact same issue)