I just noticed that there exist two same ports in the ports tree. Since math/mandelbulber2 was added later, it should be removed.
A commit references this bug: Author: yuri Date: Wed Feb 5 02:07:28 UTC 2020 New revision: 525251 URL: https://svnweb.freebsd.org/changeset/ports/525251 Log: Remove duplicate port math/mandelbulber2 graphics/mandelbulber was created earlier. PR: 243868 Reported by: pkubaj Changes: head/MOVED head/math/Makefile head/math/mandelbulber2/
Removed, thanks!