Commit error with Xapian 1.4.11

A Recoll user gets the following message while indexing:

"Attempted to delete or modify an entry in a non-existent posting list for #bannerholder"

The exception happens during a commit call. Xapian version 1.4.11, Debian Buster

A little more detail here: https://opensourceprojects.eu/p/recoll1/tickets/108/

I asked if this was reproducible, and to run the indexing in single-thread
mode to simplify the situation.

I'm not too sure if a Recoll bug could cause this, or if this has to be a
Xapian issue, I can open a ticket if more appropriate.

jf