Comment # 5 on bug 952799 from macias -

@Sa������a, I believe the title should mention ".ktc" file, not ".tc" one.

Anyway, I don't know how about -sqlite package, but -db one for sure ignores
.db file and uses .ktc one. If someone already trained BF this work is simply
lost (thus bumping it to "major" severity on the basis -- after years of usage
program running from scratch is not "some loss of functionality under specific
circumstances", it is complete loss).

I checked two setups (many thanks for help Bogofilter community).

With bogofilter-db I get the info about BF:

bogofilter-kyotocabinet version 1.2.4
    Database: Berkeley DB 4.8.30: (March 24, 2018) AUTO-XA
wordlist             r,word,/home/mist/.bogofilter/wordlist.kct,0

and with bogofilter-kct:

bogofilter-kyotocabinet version 1.2.4
    Database: Kyoto Cabinet 1.2.77 (TreeDB)
wordlist             r,word,/home/mist/.bogofilter/wordlist.kct,0

So both backends are hitting the same file.

Leap 15.3.