databases/postgis31 works perfectly on amd64, however on many i386 machines with either postgresql13-server or postgresql12-server it fails with "Undefined symbol" errors.

postgis31-3.1.5 from official "latest" package repo on a fresh FreeBSD 13.0-RELEASE-p8 i386 install:

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postgres=# CREATE DATABASE postgistest;
CREATE DATABASE
postgres=# \c postgistest
You are now connected to database "postgistest" as user "postgres".
postgistest=# CREATE EXTENSION postgis;
ERROR:  could not load library "/usr/local/lib/postgresql/postgis-3.so": /usr/local/lib/postgresql/postgis-3.so: Undefined symbol "_ZNKSt8__detail20_Prime_rehash_policy11_M_next_bktEj"
postgistest=# 
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Some searching suggests this might be related to a library mismatch between base and ports. I notice the i386 library that does not work is linking to libgcc_s.so.1 from the gcc10 port:

i386# ldd /usr/local/lib/postgresql/postgis-3.so | grep libgcc
        libgcc_s.so.1 => /usr/local/lib/gcc10/libgcc_s.so.1 (0x207a4000)
i386# 

while the working amd64 machine is linking to base libgcc_s.so.1:

amd64# ldd /usr/local/lib/postgresql/postgis-3.so | grep gcc
	libgcc_s.so.1 => /lib/libgcc_s.so.1 (0x80169b000)
amd64#

I have tried playing around with LD_LIBRARY_PATH/LD_PRELOAD but have not found a working combination as postgresql-server itself appears to need the ports gcc10 libgcc_s.so.1.

FYI this also affects databases/postgis32 on i386, although the undefined symbol is different with 3.2:

postgistest=# CREATE EXTENSION postgis;
ERROR:  could not load library "/usr/local/lib/postgresql/postgis-3.so": /usr/local/lib/postgresql/postgis-3.so: Undefined symbol "_ZNSt8ios_base4InitD1Ev"
postgistest=#