Steps to reproduce the issue

Issue occurs when multiple fields are placed within a single cell for subform.repeatable-table. Add the following after https://github.com/joomla/joomla-cms/blob/staging/modules/mod_login/mod_login.xml#L26

<field name="sebform1" type="subform" label="subform"
	layout="joomla.form.field.subform.repeatable-table" multiple="true" groupByFieldset="true">
	<form>
		<fieldset name="group1" label="group 1">
			<field type="text" name="text" label="text 1"/>
			<field type="text" name="text2" label="text 2"/>
			<field type="text" name="text3" label="text 3"/>
		</fieldset>
		<fieldset name="group2" label="group 2">
			<field type="text" name="text4" label="text 4"/>
			<field type="textarea" name="textarea" label="textarea"/>
		</fieldset>
	</form>
</field>

Open login module settings. Check repeatable field styling on reduced browser width.

Expected result

Something along the lines of...

Actual result

A mess....

Additional comments

I can fix once #20209 is merged.