A recent change to the reference counting in the javascript controlling subforms causes J3.9.15 to not properly save nested subforms. A form rendered from saved data shows blank fields as the data can not properly be read. Rendered subforms with data saved under J3.9.14 is incorrectly displayed.

###Steps to reproduce:

Install Joomla 3.9.14
Modify the file j3914/plugins/actionlog/joomla/joomla.xml to include the following configuration fields:

<config>
	<fields name="params">
		<fieldset name="basic">
			<field name="items"
				   type="subform"
				   label="Items"
				   multiple="true"
			>
				<form name="itemform">
					<field name="itemtext"
						   type="text"
						   label="Item Text"
					/>
					<field name="subitems"
						   type="subform"
						   label="Subitems"
						   multiple="true"
					>
						<form name="subitemform">
							<field name="subitemtext"
								   type="text"
								   label="Subitem Text"
							/>
						</form>
					</field>
				</form>
			</field>
		</fieldset>
	</fields>
</config>

Set the fields in the plugin config page to something like this:

	Items	
		Item text	-> Textvalue
		
		Subitems	-> 
						Subitem Text -> subitemtextvalue

Save the configuration.
From your debugger, see that the POSTed data looks like this:

jform
	params	
		items
			items0
				itemtext = "Textvalue"
				subitems
					subitems0
						subitemtext = "subitemtextvalue"

From phpmyadmin, see that the saved params json data looks like this:

{"items":{"items0":{"itemtext":"Textvalue","subitems":{"subitems0":{"subitemtext":"subitemtextvalue"}}}}}

Compare this to J3.9.15:
Install Joomla 3.9.15
Modify the file j3915/plugins/actionlog/joomla/joomla.xml to include the following configuration fields, same as 3.9.14.

<config>
	<fields name="params">
		<fieldset name="basic">
			<field name="items"
				   type="subform"
				   label="Items"
				   multiple="true"
			>
				<form name="itemform">
					<field name="itemtext"
						   type="text"
						   label="Item Text"
					/>
					<field name="subitems"
						   type="subform"
						   label="Subitems"
						   multiple="true"
					>
						<form name="subitemform">
							<field name="subitemtext"
								   type="text"
								   label="Subitem Text"
							/>
						</form>
					</field>
				</form>
			</field>
		</fieldset>
	</fields>
</config>

Set the fields in the plugin config page to something like this, same as 3.9.14.

	Items	
		Item text	-> Textvalue
		
		Subitems	-> 
						Subitem Text -> subitemtextvalue

Save the configuration.
From your debugger, see that the POSTed data looks like this, different from 3.9.14:

jform
	params	
		items
			items0
				itemtext = "Textvalue"
				subitems = ""
			items1
				subitems
					subitems0
						subitemtext = "subitemtextvalue"

From phpmyadmin, see that the J3.9.15 saved params json data looks like this, different from 3.9.14:

{"items":{"items0":{"itemtext":"Textvalue","subitems":[]},"items1":{"subitems":{"subitems0":{"subitemtext":"subitemtextvalue"}}}}}

For comparison, the J3.9.14 params data was:

{"items":{"items0":{"itemtext":"Textvalue","subitems":{"subitems0":{"subitemtext":"subitemtextvalue"}}}}}