diff --git a/runtime/lua/vim/diagnostic.lua b/runtime/lua/vim/diagnostic.lua
index d96b932b68..e01b1f2daa 100644
--- a/runtime/lua/vim/diagnostic.lua
+++ b/runtime/lua/vim/diagnostic.lua
@@ -418,7 +418,7 @@ local bufnr_and_namespace_cacher_mt = {
 }
 
 -- bufnr -> ns -> Diagnostic[]
-local diagnostic_cache = {} --- @type table<integer,table<integer,vim.Diagnostic[]>>
+local diagnostic_cache = {} --- @type table<integer,table<integer,vim.Diagnostic[]?>>
 do
   local group = api.nvim_create_augroup('nvim.diagnostic.buf_wipeout', {})
   setmetatable(diagnostic_cache, {
@@ -710,16 +710,6 @@ local function norm_diag(bufnr, namespace, d)
   d1.bufnr = bufnr
 end
 
---- @param namespace integer
---- @param bufnr integer
---- @param diagnostics vim.Diagnostic.Set[]
-local function set_diagnostic_cache(namespace, bufnr, diagnostics)
-  for _, diagnostic in ipairs(diagnostics) do
-    norm_diag(bufnr, namespace, diagnostic)
-  end
-  diagnostic_cache[bufnr][namespace] = diagnostics
-end
-
 --- @param bufnr integer
 --- @param last integer
 local function restore_extmarks(bufnr, last)
@@ -1260,10 +1250,16 @@ function M.set(namespace, bufnr, diagnostics, opts)
 
   bufnr = vim._resolve_bufnr(bufnr)
 
+  for _, diagnostic in ipairs(diagnostics) do
+    norm_diag(bufnr, namespace, diagnostic)
+  end
+
+  --- @cast diagnostics vim.Diagnostic[]
+
   if vim.tbl_isempty(diagnostics) then
     diagnostic_cache[bufnr][namespace] = nil
   else
-    set_diagnostic_cache(namespace, bufnr, diagnostics)
+    diagnostic_cache[bufnr][namespace] = diagnostics
   end
 
   M.show(namespace, bufnr, nil, opts)
@@ -1272,7 +1268,7 @@ function M.set(namespace, bufnr, diagnostics, opts)
     modeline = false,
     buffer = bufnr,
     -- TODO(lewis6991): should this be deepcopy()'d like they are in vim.diagnostic.get()
-    data = { diagnostics = diagnostic_cache[bufnr][namespace] },
+    data = { diagnostics = diagnostics },
   })
 end
 
diff --git a/test/functional/lua/diagnostic_spec.lua b/test/functional/lua/diagnostic_spec.lua
index ad08a33fc0..7f94d0ed54 100644
--- a/test/functional/lua/diagnostic_spec.lua
+++ b/test/functional/lua/diagnostic_spec.lua
@@ -2714,6 +2714,23 @@ describe('vim.diagnostic', function()
         eq({}, result)
       end
     end)
+
+    it('always passes a table to DiagnosticChanged autocommand', function()
+      local result = exec_lua(function()
+        local changed_diags --- @type vim.Diagnostic[]?
+        vim.api.nvim_create_autocmd('DiagnosticChanged', {
+          buffer = _G.diagnostic_bufnr,
+          callback = function(args)
+            --- @type vim.Diagnostic[]
+            changed_diags = args.data.diagnostics
+          end,
+        })
+        vim.diagnostic.set(_G.diagnostic_ns, _G.diagnostic_bufnr, {})
+        return changed_diags
+      end)
+      eq('table', type(result))
+      eq(0, #result)
+    end)
   end)
 
   describe('open_float()', function()