I’ve worked out what’s causing this
the wp_{prefix}wfconfig table has name, val pairs
and it seems that there’s a large binary blob that is stored as name=’wfsd_engine’, on my system it’s 811675 bytes long.
And it’s frequently appended to this, and it never changes.
I had 447 identical entries in there, which might be a daily update causing it.
By the look of the code, it never looks at more than one row when it calls get_ser().
I deleted 446 of the surplus identical copies of this with no obvious ill-effect
in my example, it was
delete from wp_{prefix}_wfconfig where name=’wfsd_engine’
limit 446
and it appeared to have no ill-effect, but it doesn’t stop the problem re-occurring.