Steps to reproduce the issue

Create a user required custom field named "test"
Show this field on registration page
Include this field in the new user mailtemplate using the code {COM_FIELDS}test{/COM_FIELDS}
Register an new user account

Expected result

When user creates a new account, this placeholder in the email should be replaced with submitted value

Actual result

The placeholder is replaced with a empty string

System information (as much as possible)

/libraries/src/Mail/MailTemplate.php
Function replaceTags handles the replacements

on line 354-360 it is missing the code to supply the replacement if this is not an array:

Additional comments

Change this code for a fix

foreach ($value as $subvalue)
{
	if (is_array($subvalue))
	{
		$replacement .= $this->replaceTags($matches[1][$i], $subvalue);
	}
}

to

foreach ($value as $name => $subvalue)
{
	if (is_array($subvalue))
	{
		$replacement .= $this->replaceTags($matches[1][$i], $subvalue);
	}
	elseif (is_string($subvalue) && $name == $matches[1][$i])
	{
		$replacement .= $subvalue;
	}
}