This is the best. Especially when the password is being autotyped by the pw manager and so you never see the truncation and now have a bad pw saved in your manager. Alongside a restrictive password policy with no ui explaining what the policy is.
This happens on some HP printers too, the web interface lets you happily enter lengthy passwords, but doesn't bother telling you it truncated the entry at 16 or 12 characters.
I unfortunately had the infuriating experience dealing with a (government, of course) site that did this. To add to the experience, not only did it silently truncate at registration, but it did NOT truncate on the login fields. And of course, it has a lockout after several failed attempts. UX gore at it's finest.