Thank you for the report. This took a while to track down, as I originally thought this was FreeBSD related, but it turns out this was because of a patch applied to Postgres 9.5.22 that loosened the legal specification for the circle type, such that the previous failing "SELECT '(12,34)'::circle" is now working.
Should be fixed as of d38a52f14ac18968dc2b4319f7b826c4218ebbfc, and will be a part of the next release.