Steps to reproduce the issue

The issue occurs when you have more than one record to bind and save.
eg.

foreach($recordset as $record)
{
	$table->save($record);
}

If the record contains fields that admit null and that are set to null, they don't get bound properly by the bind() function.
The first record to be saved works properly, because the Table object is freshly instantiated and initialized.
During subsequent bind() operations, fields containing null values don't overwrite table properties already set to non-null.

I created a new PHPunit method in \tests\Integration\Libraries\Cms\Table\TableTest.php, to better explain the issue

/**
 * Test that bind() properly bind a null field 
 *
 * @return  void
 * @since   4.1.0
 */	public function testBindNullFieldsWithoutReset()
{
	$data_1 = [
		'title' => 'Title One',
		'checked_out' => 1
	];
	
	$data_2 = [
		'title' => 'Title Two',
		'checked_out' => null
	];
	
	// Check if nullable int property is 1
	$this->object->bind($data_1);
	$this->assertEquals(1, $this->object->checked_out);
	
	// Check if nullable int property is null
	$this->object->bind($data_2);
	$this->assertEquals(null, $this->object->checked_out);
}

Expected result

When I run the test above, I expect that bind($data_2) sets 'checkout' property to null.

Actual result

The actual result is that the 'checkout' property remains set to 1.

System information (as much as possible)

PHP Version 8.0.19
Web Server Apache/2.4.38 (Debian)
WebServer to PHP Interface fpm-fcgi
Joomla! Version Joomla! 4.1.0 Stable [ Kuamini ] 15-February-2022 09:30 GMT

Additional comments (proposed solution)

I traced the root of the problem to line 695 of file libraries\src\Table\Table.php

	// Bind the source value, excluding the ignored fields.
	foreach ($this->getProperties() as $k => $v)
	{
		// Only process fields not in the ignore array.
		if (!\in_array($k, $ignore)) 
		{
			if (isset($src[$k])) //line 695
			{
				$this->$k = $src[$k];
			}
		}
	}

The isset() function does return false if $src[$k] exists, but contains null.

See:
https://www.php.net/manual/en/function.isset.php

isset() will return false when checking a variable that has been assigned to null. Also note that a null character ("\0") is not equivalent to the PHP null constant.

By using the 'array_key_exists' function, the existence of key $k in the $src array is properly assessed even when its value is null.

Suggested change:

	// Bind the source value, excluding the ignored fields.
	foreach ($this->getProperties() as $k => $v)
	{
		// Only process fields not in the ignore array.
		if (!\in_array($k, $ignore)) 
		{
			if(array_key_exists($k, $src)) //new line 695
			{
				$this->$k = $src[$k];
			}
		}
	}

Note:
There's also a workround: call a reset() before each save() to force the re-initialization of the table object properties. But it's wasteful.