This bug occurs on Apex 21.2.10, but it has been fixed in 23.2.4 (or somewhere in between).

The "Warn on Unsaved Changes" option in Apex is very useful to prevent a user from losing data they enter on a page.
From the built-in Help: "Specify if the user is warned when trying to navigate away from the page, where the page contains unsaved changes. Page Items and Buttons also offer more granular control over when this check is performed, and what is included in it. For Buttons, you can specify if clicking the button will perform the check. For Page Items, you can specify if changes to the item are included, or ignored by the check (see the Warn on Unsaved Changes attribute for both).
Note: This does not apply to an end user closing a Modal dialog page."

However, in older Apex versions it may not always behave as you expect.
In this example, try changing item 1 manually and then click "Home" (to navigate to another page). Your browser will show you a warning.
Then click "Reset", and click "Fill with Set Value", and then click "Home". Same result, your browser will show you a warning.
Finally, click "Reset", and click "Fill with Modal Dialog", select a value from the select list (which closes the modal dialog), and then click "Home". This time your browser will NOT show you a warning.

As stated above, you will not see the bug on this site because I'm running a newer Apex version.
To see the bug in action, rebuild this page in Apex 21.2.10 (or older versions, no idea when this was introduced or fixed).

Some links with related info:
https://forums.oracle.com/ords/apexds/post/cancel-button-on-modal-page-warn-on-unsaved-changes-5114
https://zerointegration.com/apex/oracle-apex-warn-unsaved-changes-for-custom-fields/
https://forums.oracle.com/ords/apexds/post/issue-with-warn-on-unsaved-changes-8718
https://docs.oracle.com/en/database/oracle/apex/23.2/aexjs/apex.page.html#.warnOnUnsavedChanges
https://gist.github.com/hhudson/c2d57dcbdbbc7e01021aada718fe4506#file-apex-detect-changed-items-js

Some JavaScript to find all page items that would trigger the warning (if all the Page and Button "Warn on Unsaved Changes" settings are correct) - run this in the browser console: allItems = apex.page.forEachPageItem; allItems( $( "#wwvFlowForm" ), function( el, name ) { if ( apex.item(name).isChanged() && !apex.item(name).element[0].classList.value.includes("js-ignoreChange") ) { console.log("Item with unsaved changes: " + name); } }, true );

Page

Identification
	Page ID	1179
	Name	Apex - General - Warn on Unsaved Changes
	Page Alias	APEX-GENERAL-WARN-ON-UNSAVED-CHANGES
Navigation
	Warn on Unsaved Changes	Yes

Item

Identification
	Sequence	10
	Name	P1179_ITEM_1
	Display As	Text Field
User Interface
	Region	Warn on Unsaved Changes
Advanced
	Warn on Unsaved Changes	Page Default

Button

Name
	Button Name	FILL_WITH_SET_VALUE
	Text Label / Alt	Fill with Set Value
Displayed
	Sequence	30
	Display in Region	Warn on Unsaved Changes
Behavior
	Action	Defined by Dynamic Action
	Execute Validations	Yes

Button

Name
	Button Name	RESET
	Text Label / Alt	Reset
Displayed
	Sequence	20
	Display in Region	Warn on Unsaved Changes
Behavior
	Action	Redirect to Page in this Application
	Target (Redirect URL)	f?p=&APP_ID.:&APP_PAGE_ID.:&SESSION.::&DEBUG.:&APP_PAGE_ID.::
	Execute Validations	Yes

Dynamic Action

Identification
	Name	Fill with Set Value
Execution Options
	Sequence	10
When
	Event	Click
	Selection Type	Button
	Button	FILL_WITH_SET_VALUE
Advanced
	Event Scope	Static
True Action
Identification
	Action	Set Value
Settings
	Set Type	PLSQL_EXPRESSION
	PL/SQL Expression	systimestamp
	Escape Special Characters	Y
	Suppress Change Event	N
Execution Options
	Sequence	10
	Fire On Initialization	No
Affected Elements
	Selection Type	Item
	Item(s)	P1179_ITEM_1

Dynamic Action

Identification
	Name	Fill with Modal Dialog
Execution Options
	Sequence	20
When
	Event	Dialog Closed
	Selection Type	Button
	Button	FILL_WITH_MODAL_DIALOG
Advanced
	Event Scope	Static
True Action
Identification
	Action	Set Value
Settings
	Set Type	DIALOG_RETURN_ITEM
	Return Item	P1180_ITEM
	Suppress Change Event	N
Execution Options
	Sequence	10
	Fire On Initialization	No
Affected Elements
	Selection Type	Item
	Item(s)	P1179_ITEM_1

Item

Note: This object is located on page 1180

Identification
	Sequence	10
	Name	P1180_ITEM
	Display As	Select List
User Interface
	Region	Fill Item
Advanced
	Warn on Unsaved Changes	Page Default

Dynamic Action

Note: This object is located on page 1180

Identification
	Name	onChange P1180_ITEM Close Dialog
Execution Options
	Sequence	10
When
	Event	Change
	Selection Type	Item
	Item(s)	P1180_ITEM
Advanced
	Event Scope	Static
True Action
Identification
	Action	Close Dialog
Settings
	Items to Return	P1180_ITEM
Execution Options
	Sequence	10
	Fire On Initialization	No