Use the github hosted MQTT doxygen to see if that fixes the issue

docs/doxygen/config.doxyfile

@@ -2368,7 +2368,7 @@ SKIP_FUNCTION_MACROS   = YES
 # the path). If a tag file is not located in the directory in which doxygen is
 # run, you must also specify the path to the tagfile here.
 
-TAGFILES               = source/dependency/coreMQTT/docs/doxygen/output/mqtt.tag=https://freertos.org/Documentation/api-ref/coreMQTT/docs/doxygen/output/html
+TAGFILES               = source/dependency/coreMQTT/docs/doxygen/output/mqtt.tag=https://freertos.github.io/coreMQTT/v2.1.0/index.html
 
 # When a file name is specified after GENERATE_TAGFILE, doxygen will create a
 # tag file that is based on the input files it reads. See section "Linking to