works

root cause identified

platformio.ini

@@ -195,6 +195,7 @@ board         = teensy20pp
 build_flags   = ${common.build_flags} -I $BUILDSRC_DIR
 #board_f_cpu  = 20000000L     ; Bug in Arduino framework disallows boards running at 20Mhz
 lib_deps      = ${common.lib_deps}
+lib_ldf_mode  = deep+
 src_filter    = ${common.default_src_filter}
 
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