libheif 1.18.0: Test suite fails with encode and region tests even with LIBHEIF_PLUGIN_PATH set

[ellenjohnson]

Hello!
On Linux, I'm building libheif v1.18.0 with libde265 as a dynamic plugin:

WITH_LIBDE265:BOOL=ON
WITH_LIBDE265_PLUGIN:BOOL=ON

The build runs fine, but the tests fail -- only two tests are run, encode and region - and they both fail similar to the descriptions in #986 and #965.
I see solution is to set LIBHEIF_PLUGIN_PATH.
Does this mean set it to the absolute path where the plugins are built during the build process, meaning libheif/plugins which contains (amongst the decoder/coder c files) the libheif-libde265.so? Or the absolute path to the location of the libde265.so that I built when building libde265 v1.0.15 (prior to building libheif)?
When I set it to the libheif/plugins path, I get the following results (just showing results for encode test for brevity):

Start testing: Aug 22 10:21 EDT
----------------------------------------------------------
1/2 Testing: encode
1/2 Test: encode
Command: "/mathworks/devel/sandbox/ellenj/3p-tmw/3p/derived/glnxa64/libheif/tests/encode"
Directory: /mathworks/devel/sandbox/ellenj/3p-tmw/3p/derived/glnxa64/libheif/tests
"encode" start time: Aug 22 10:21 EDT
Output:
----------------------------------------------------------

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
encode is a Catch v2.13.9 host application.
Run with -? for options

-------------------------------------------------------------------------------
ispe odd size
-------------------------------------------------------------------------------
/mathworks/devel/sandbox/ellenj/3p-tmw/3p/derived/glnxa64/libheif/tests/encode.cc:165
...............................................................................

/mathworks/devel/sandbox/ellenj/3p-tmw/3p/derived/glnxa64/libheif/tests/encode.cc:140: FAILED:
  REQUIRE( err.code == heif_error_Ok )
with expansion:
  3 == 0

===============================================================================
test cases: 1 | 1 failed
assertions: 4 | 3 passed | 1 failed

<end of output>
Test time =   0.01 sec
----------------------------------------------------------
Test Failed.
"encode" end time: Aug 22 10:21 EDT
"encode" time elapsed: 00:00:00
----------------------------------------------------------

When I set it to the path to containing libde265.so from building libde265, I get glibc errors:

Start testing: Aug 22 11:00 EDT 
----------------------------------------------------------
1/2 Testing: encode
1/2 Test: encode
Command: "/mathworks/devel/sandbox/ellenj/3p-tmw/3p/derived/glnxa64/libheif/tests/encode"
Directory: /mathworks/devel/sandbox/ellenj/3p-tmw/3p/derived/glnxa64/libheif/tests
"encode" start time: Aug 22 11:00 EDT 
Output:
----------------------------------------------------------
/mathworks/devel/sandbox/ellenj/3p-tmw/3p/derived/glnxa64/libheif/tests/encode: /lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.29' not found (required by /mathworks/devel/sandbox/ellenj/3p-tmw/3p/derived/glnxa64/libheif/tests/encode)
`/mathworks/devel/sandbox/ellenj/3p-tmw/3p/derived/glnxa64/libheif/tests/encode:` /lib/x86_64-linux-gnu/libstdc++.so.6: version `CXXABI_1.3.13' not found (required by /mathworks/devel/sandbox/ellenj/3p-tmw/3p/derived/glnxa64/libheif/tests/encode)
/mathworks/devel/sandbox/ellenj/3p-tmw/3p/derived/glnxa64/libheif/tests/encode: /lib/x86_64-linux-gnu/libstdc++.so.6: version `CXXABI_1.3.13' not found (required by /mathworks/devel/sandbox/ellenj/3p-tmw/3p/derived/glnxa64/libheif/libheif/libheif.so.1)
/mathworks/devel/sandbox/ellenj/3p-tmw/3p/derived/glnxa64/libheif/tests/encode: /lib/x86_64-linux-gnu/libstdc++.so.6: version `GLIBCXX_3.4.29' not found (required by /mathworks/devel/sandbox/ellenj/3p-tmw/3p/derived/glnxa64/libheif/libheif/libheif.so.1)
<end of output>
Test time =   0.01 sec 
----------------------------------------------------------
Test Failed.
"encode" end time: Aug 22 11:00 EDT 
"encode" time elapsed: 00:00:00

I'm not sure what I'm doing wrong here, any help is appreciated!