Hmm, howm 1.4.6 seems to work on Emacs 27.1 for me at a glance.

(1)
Would you try M-x honest-report RET and paste "Emacs version" part?

My case:
[Emacs] 27.1 (x86_64-pc-linux-gnu) of 2020-09-13
[system] gnu/linux
[window system] x
[ENV] LC_ALL=nil, LC_CTYPE=nil, LANGUAGE=ja, LANG=ja_JP.ujis

(2)
Can you find the difference between OK keywords and NG keywords?

(3)
Are you using (setq howm-view-use-grep t) ?
If so,
  M-x describe-variable RET howm-call-process-last-command RET
will show the last grep command.
You can reproduce it by pasting
  (apply ’howm-call-process howm-call-process-last-command)
into *scratch* buffer and type C-j just after ")".
Do you notice anything?

(4)
If possible, would you try to make a minimum set of
.emacs, ~/.howm-keys, and ~/howm/* together with a minimum
sequence of operations to reproduce your issue?

-- 
khi at users.osdn.me