diff --git a/packages/app-cli/tests/html_to_md/sub_sup_insert_strikethrough.html b/packages/app-cli/tests/html_to_md/sub_sup_insert_strikethrough.html
index d1149f97cc..10db976c29 100644
--- a/packages/app-cli/tests/html_to_md/sub_sup_insert_strikethrough.html
+++ b/packages/app-cli/tests/html_to_md/sub_sup_insert_strikethrough.html
@@ -1 +1 @@
-X<sub>1</sub> X<sup>1</sup> <ins>Insert</ins> <s>Strike</s>
\ No newline at end of file
+X<sub>1</sub> X<sup>1</sup> <ins>Insert</ins> <span style="text-decoration: underline;">Insert alt</span> <s>Strike</s>
\ No newline at end of file
diff --git a/packages/app-cli/tests/html_to_md/sub_sup_insert_strikethrough.md b/packages/app-cli/tests/html_to_md/sub_sup_insert_strikethrough.md
index 2a17e25d65..2f15f103bc 100644
--- a/packages/app-cli/tests/html_to_md/sub_sup_insert_strikethrough.md
+++ b/packages/app-cli/tests/html_to_md/sub_sup_insert_strikethrough.md
@@ -1 +1 @@
-X<sub>1</sub> X<sup>1</sup> <ins>Insert</ins> ~~Strike~~
\ No newline at end of file
+X<sub>1</sub> X<sup>1</sup> <ins>Insert</ins> <ins>Insert alt</ins> ~~Strike~~
\ No newline at end of file
diff --git a/packages/turndown/src/commonmark-rules.js b/packages/turndown/src/commonmark-rules.js
index fda46b78aa..475bf03c8e 100644
--- a/packages/turndown/src/commonmark-rules.js
+++ b/packages/turndown/src/commonmark-rules.js
@@ -1,4 +1,4 @@
-import { repeat, isCodeBlockSpecialCase1, isCodeBlockSpecialCase2, isCodeBlock } from './utilities'
+import { repeat, isCodeBlockSpecialCase1, isCodeBlockSpecialCase2, isCodeBlock, getStyleProp } from './utilities'
 const Entities = require('html-entities').AllHtmlEntities;
 const htmlentities = (new Entities()).encode;
 
@@ -73,7 +73,15 @@ rules.highlight = {
 // HTML to avoid any ambiguity.
 
 rules.insert = {
-  filter: 'ins',
+  filter: function (node, options) {
+    // TinyMCE represents this either with an <INS> tag (when pressing the
+    // toolbar button) or using style "text-decoration" (when using shortcut
+    // Cmd+U)
+    //
+    // https://github.com/laurent22/joplin/issues/5480
+    if (node.nodeName === 'INS') return true;
+    return getStyleProp(node, 'text-decoration') === 'underline';
+  },
 
   replacement: function (content, node, options) {
     return '<ins>' + content + '</ins>'
diff --git a/packages/turndown/src/utilities.js b/packages/turndown/src/utilities.js
index d3da0dc839..0554f00804 100644
--- a/packages/turndown/src/utilities.js
+++ b/packages/turndown/src/utilities.js
@@ -78,3 +78,16 @@ export function isCodeBlock(node) {
     node.firstChild.nodeName === 'CODE'
   )
 }
+
+export function getStyleProp(node, name) {
+  const style = node.getAttribute('style');
+  if (!style) return null;
+
+  name = name.toLowerCase();
+  if (!style.toLowerCase().includes(name)) return null;
+
+  const o = css.parse('div {' + style + '}');
+  if (!o.stylesheet.rules.length) return null;
+  const prop = o.stylesheet.rules[0].declarations.find(d => d.property.toLowerCase() === name);
+  return prop ? prop.value : null;
+}