It is probably better to prepend ranther than append

init.zsh

@@ -27,7 +27,7 @@ load_zim_function() {
   local mod_function
 
   # autoload searches fpath for function locations; add enabled module function paths
-  fpath+=(${argv:+${ZIM}/modules/${^zmodules}/functions(/FN)})
+  fpath=(${argv:+${ZIM}/modules/${^zmodules}/functions(/FN)} ${fpath})
 
   function {
     setopt LOCAL_OPTIONS EXTENDED_GLOB

modules/completion/init.zsh

@@ -13,7 +13,7 @@ if [[ ${TERM} == 'dumb' ]]; then
 fi
 
 # add the completions to the fpath
-fpath+=(${0:h}/external/src)
+fpath=(${0:h}/external/src ${fpath})
 
 # load and initialize the completion system
 autoload -Uz compinit && compinit -C -d ${ZDOTDIR:-$HOME}/.zcompdump