Same on Russian forums.

Some topic slugs cannot be urldecoded correctly due to being cut in the middle of a UTF-8 sequence, but somehow still work:

• ​https://ru.wordpress.org/support/topic/%d0%a1%d0%b2%d0%be%d1%80%d0%be%d0%b2%d0%b0%d0%bb%d0%b8-%d0%bd%d0%b0%d1%88%d1%83-%d1%80%d0%b0%d0%b7%d1%80%d0%b0%d0%b1%d0%be%d1%82%d0%ba%d1%83-%d0%b8-%d0%b2%d1%8b%d0%bb%d0%be%d0%b6%d0%b8%d0%bb%d0%b8-%d0/
• ​https://ru.wordpress.org/support/topic/%d0%9a%d0%b0%d0%ba-%d0%bf%d0%be%d0%bc%d0%b5%d0%bd%d1%8f%d1%82%d1%8c-%d0%bc%d0%b5%d1%81%d1%82%d0%b0%d0%bc%d0%b8-%d1%81%d0%b0%d0%b9%d0%b4%d0%b1%d0%b0%d1%80-%d0%b8-%d0%be%d1%81%d0%bd%d0%be%d0%b2%d0%bd%d0/

Others are completely broken and produce a 400 Bad Request error:

• ​https://ru.wordpress.org/support/topic/%d0%ba%d0%b0%d0%ba-%d0%be%d1%82%d0%ba%d0%bb%d1%8e%d1%87%d0%b8%d1%82%d1%8c-%d1%8d%d0%ba%d1%80%d0%b0%d0%bd%d0%b8%d1%80%d0%be%d0%b2%d0%b0%d0%bd%d0%b8%d0%b5-%d0%ba%d0%b8%d1%80%d0%b8%d0%bb%d0%bb%d0%b8%d1%8/
• ​https://ru.wordpress.org/support/topic/%D0%9D%D0%B0-%D0%BF%D0%BB%D0%B0%D1%82%D0%BD%D0%BE%D0%BC-%D1%88%D0%B0%D0%B1%D0%BB%D0%BE%D0%BD%D0%B5-%D0%BC%D0%BE%D0%B6%D0%BD%D0%BE-%D0%B4%D0%B5%D0%BB%D0%B0%D1%82%D1%8C-%D0%BC%D0%BD%D0%BE%D0%B3%D0%BE-%d/

@jmdodd: Is there a way to find and fix all the affected topics?

Is it possible to change the permalink settings to ID based rather than slug ones? I suspect that this is caused by MySQL column length limitation. WP core has the same problem.

See: ​https://core.trac.wordpress.org/ticket/17771

I've run a utf8-friendly fixer script on the post_names for each already-imported bbPress forum, and will adjust the import script going forward. The above invalid links are no longer used as topic links -- the topics are now correctly linked at:

​https://ru.wordpress.org/support/topic/%D0%BA%D0%B0%D0%BA-%D0%BE%D1%82%D0%BA%D0%BB%D1%8E%D1%87%D0%B8%D1%82%D1%8C-%D1%8D%D0%BA%D1%80%D0%B0%D0%BD%D0%B8%D1%80%D0%BE%D0%B2%D0%B0%D0%BD%D0%B8%D0%B5-%D0%BA%D0%B8%D1%80%D0%B8%D0%BB%D0%BB%D0%B8/
​https://ru.wordpress.org/support/topic/%D0%BD%D0%B0-%D0%BF%D0%BB%D0%B0%D1%82%D0%BD%D0%BE%D0%BC-%D1%88%D0%B0%D0%B1%D0%BB%D0%BE%D0%BD%D0%B5-%D0%BC%D0%BE%D0%B6%D0%BD%D0%BE-%D0%B4%D0%B5%D0%BB%D0%B0%D1%82%D1%8C-%D0%BC%D0%BD%D0%BE%D0%B3%D0%BE/

Replying to tenpura:

I suspect that this is caused by MySQL column length limitation. WP core has the same problem.

See: ​https://core.trac.wordpress.org/ticket/17771

It's worth noting that ​#wp17771 only affects pingbacks, regular post slugs are truncated correctly since 3.6 (see ​#wp21013).

This topic URL works, but cannot be urldecoded correctly due to an extra %d1 at the end, so browsers don't display it in human-readable form in the address bar, like other topics:
​https://ru.wordpress.org/support/topic/%d0%94%d0%b2%d1%83%d1%8f%d0%b7%d1%8b%d1%87%d0%bd%d1%8b%d0%b9-%d1%81%d0%b0%d0%b9%d1%82-%d0%b1%d0%b5%d0%b7-%d0%b8%d1%81%d0%bf%d0%be%d0%bb%d1%8c%d0%b7%d0%be%d0%b2%d0%b0%d0%bd%d0%b8%d1%8f-%d0%bc%d0%b0%d1/

Any way to double-check the fixer script?

I re-ran the script with an additional set of decode/encode/decode comparisons and it caught this instance but no others:

Updated post ID 216625 from 
%d0%94%d0%b2%d1%83%d1%8f%d0%b7%d1%8b%d1%87%d0%bd%d1%8b%d0%b9-%d1%81%d0%b0%d0%b9%d1%82-%d0%b1%d0%b5%d0%b7-%d0%b8%d1%81%d0%bf%d0%be%d0%bb%d1%8c%d0%b7%d0%be%d0%b2%d0%b0%d0%bd%d0%b8%d1%8f-%d0%bc%d0%b0%d1
to
%d0%94%d0%b2%d1%83%d1%8f%d0%b7%d1%8b%d1%87%d0%bd%d1%8b%d0%b9-%d1%81%d0%b0%d0%b9%d1%82-%d0%b1%d0%b5%d0%b7-%d0%b8%d1%81%d0%bf%d0%be%d0%bb%d1%8c%d0%b7%d0%be%d0%b2%d0%b0%d0%bd%d0%b8%d1%8f-%d0%bc%d0%b0

I'll leave this open until I'm certain that I've fixed the importer.