locate – plocate -b not searching multiple databases

Ubuntu 21.04. I want to search multiple databases (created for external drives) using plocate:

plocate -d ~/.db1.db:~/.db2.db: -i -r "SEARCH_TERM"

Whenever I run that using queries that I know have matching results within db2, I get results that instead end with (after the listing of matches from db1):

~/.db2.db: No such file or directory

That is reported even if I swap the order of the two dbs around, i.e. 2 & 1 instead of 1 & 2. In that case it reports:

~/.db1.db: No such file or directory

even though I’d run it just before prior to the swap and found matches in db1.

That is even the case if I specify the same db twice, i.e. db1 & db1 or db2 & db2. First it lists the matches from dbX, then at the end it reports no such file or directory for dbX.

Man pages for plocate such as here: https://plocate.sesse.net/plocate.1.html
indicate that the colon character can be used to separate different databases previously created using updatedb. I also found suggestions on the web that the trailing colon with nothing after it ensured that the default db would also be included/searched.

If I give only the one custom db:

plocate -d ~/.db1.db: -i -r "SEARCH_TERM"

then I get a similar result (after the matches from db1):

No such file or directory

which I presume relates to the default db?

The behaviour observed seems to suggest that only the first db is actually being searched, and that any second db listed produces an error, even if there is also a third db listed. I checked that by putting the default db in the middle:

plocate -d ~/.db1.db::~/.db2.db -i -r "$SEARCH_TERM"

and again I got the error message seemingly corresponding to the default db:

No such file or directory

and matches only found from db1 but not db2.