diff --git a/src/nvim/insexpand.c b/src/nvim/insexpand.c
index f4e8f21b74..d7375fd9ee 100644
--- a/src/nvim/insexpand.c
+++ b/src/nvim/insexpand.c
@@ -1418,16 +1418,33 @@ static String *get_leader_for_startcol(compl_T *match, bool cached)
     return NULL;
   }
 
-  if (cpt_sources_array == NULL || compl_leader.data == NULL) {
+  if (cpt_sources_array == NULL) {
     goto theend;
   }
 
   int cpt_idx = match->cp_cpt_source_idx;
-  if (cpt_idx < 0 || compl_col <= 0) {
+  if (cpt_idx < 0) {
     goto theend;
   }
   int startcol = cpt_sources_array[cpt_idx].cs_startcol;
 
+  if (compl_leader.data == NULL) {
+    // When leader is not set (e.g. 'autocomplete' first fires before
+    // compl_leader is initialised), fall back to compl_orig_text for
+    // matches starting at or after compl_col.  Matches starting before
+    // compl_col carry pre-compl_col text and must not be compared with
+    // compl_orig_text, so return &compl_leader (NULL string) to signal
+    // "pass through" (no prefix filter).
+    if (startcol < 0 || startcol >= compl_col) {
+      return &compl_orig_text;
+    }
+    return &compl_leader;  // pass through (startcol < compl_col)
+  }
+
+  if (compl_col <= 0) {
+    goto theend;
+  }
+
   if (startcol >= 0 && startcol < compl_col) {
     int prepend_len = compl_col - startcol;
     int new_length = prepend_len + (int)compl_leader.size;
diff --git a/test/old/testdir/test_ins_complete.vim b/test/old/testdir/test_ins_complete.vim
index 15e7964c45..68d0b9facb 100644
--- a/test/old/testdir/test_ins_complete.vim
+++ b/test/old/testdir/test_ins_complete.vim
@@ -5598,10 +5598,13 @@ func Test_autocomplete_trigger()
   call assert_equal(['fooze', 'faberge'], b:matches->mapnew('v:val.word'))
 
   " Test 9: Trigger autocomplete immediately upon entering Insert mode
+  " 'faberge' is filtered out because it doesn't start with the current prefix
+  " 'foo'; non-prefix omnifunc matches are excluded from the PUM when leader
+  " is NULL (compl_orig_text is used as a fallback filter).
   call feedkeys("Sprefix->foo\<Esc>a\<F2>\<Esc>0", 'tx!')
-  call assert_equal(['foobar', 'fooze', 'faberge'], b:matches->mapnew('v:val.word'))
+  call assert_equal(['foobar', 'fooze'], b:matches->mapnew('v:val.word'))
   call feedkeys("Sprefix->fooxx\<Esc>hcw\<F2>\<Esc>0", 'tx!')
-  call assert_equal(['foobar', 'fooze', 'faberge'], b:matches->mapnew('v:val.word'))
+  call assert_equal(['foobar', 'fooze'], b:matches->mapnew('v:val.word'))
 
   bw!
   call Ntest_override("char_avail", 0)
@@ -6309,4 +6312,45 @@ func Test_helptags_autocomplete_timeout()
   bw!
 endfunc
 
+func Test_autocomplete_preinsert_null_leader()
+  " Test that non-prefix matches from omnifunc are filtered when leader is NULL.
+  " When autocomplete first fires, compl_leader is NULL.  Previously the prefix
+  " filter was bypassed, allowing non-prefix fuzzy matches to be incorrectly
+  " shown in the PUM and preinserted.
+  func NonPrefixOmni(findstart, base)
+    if a:findstart
+      return col(".") - 1
+    endif
+    " Return "key" (doesn't start with 'y') and "yellow" (starts with 'y').
+    " Simulates what a fuzzy omnifunc returns (e.g. vimcomplete#Complete with
+    " wildoptions=fuzzy).
+    return ["key", "yellow"]
+  endfunc
+
+  call Ntest_override("char_avail", 1)
+  new
+  set omnifunc=NonPrefixOmni complete=o
+  set completeopt=preinsert autocomplete
+
+  func GetState()
+    let g:line = getline('.')
+    let g:col = col('.')
+    let g:matches = complete_info(['matches']).matches->mapnew('v:val.word')
+  endfunc
+  inoremap <buffer> <F5> <C-R>=GetState()<CR>
+
+  " Type 'y': "key" should be filtered out (doesn't start with 'y'),
+  " "yellow" should be the only PUM entry and preinserted with cursor after 'y'.
+  call feedkeys("iy\<F5>\<C-E>\<Esc>", 'tx')
+  call assert_equal("yellow", g:line)
+  call assert_equal(2, g:col)
+  call assert_equal(['yellow'], g:matches)
+
+  bw!
+  set omnifunc& complete& completeopt& autocomplete&
+  call Ntest_override("char_avail", 0)
+  delfunc NonPrefixOmni
+  delfunc GetState
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab nofoldenable