Found where is the issue but I don't know how to solve.
The button itself has a class made up with the icon class

It should only have btn btn-primary

This comes from the renderButton() method in /libraries/src/Toolbar/ToolbarButton.php
$options['btnClass'] = 'button-' . $this->getName() . ' ' . ($options['btnClass'] ?? '');

where $this->getName() = the icon class defined in the HtmlView
ToolbarHelper::custom('template.cropImage', 'fas fa-crop', 'fas fa-arrows-alt', 'COM_TEMPLATES_BUTTON_CROP', false);

Other instances of ToolbarHelper::custom( do not have this issue.

@N6REJ
Any idea?

Ok, found it.

The issue is that this is the ONLY HtmlView using directly some fas fa-stuff as icon for quite a few toolbar buttons.
All other use wordings which correspond to icons.
Example here envelope:
ToolbarHelper::custom('mail.send', 'envelope.png', 'send_f2.png', 'COM_USERS_TOOLBAR_MAIL_SEND_MAIL', false);
will use as button class:
<button class="button-envelope btn btn-primary" type="button">

One solution is to modify the custom method

	public static function custom($task = '', $icon = '', $iconOver = '', $alt = '', $listSelect = true, $formId = null)
	{
		$bar = Toolbar::getInstance('toolbar');

		// Strip extension.
		$icon = preg_replace('#\.[^.]*$#', '', $icon);
		$icon = str_replace('fas fa-', '', $icon);  // ADD THIS LINE to STILL GET A CUSTOM CLASS

		// Add a standard button.
		$bar->appendButton('Standard', $icon, $alt, $task, $listSelect, $formId);
	}

to get
<button class="button-crop btn btn-primary" type="button">

What do you think?

I'm having a hard time thinking clearly today but my first question is why are we using .png for class's? Did we plan on using images then change our mind? Whats the real purpose?

It is a remnant of older code. 2.5.0 and 3.0+ have these images in the images folder of the template. It became unnecessary when we started to use icomoon.
In 2.5 they were used in the css
.icon-32-featured { background-image: url(../images/toolbar/icon-32-featured.png); }

Instead of getting rid of these (the images could be used for B/C and were kept in the template), was coded $icon = preg_replace('#\.[^.]*$#', '', $icon); for .png not to be present in the button class.

I guess we can safely get rid of these now as these images are not provided anymore in 4.0 template.

Anyhow, this can be for another patch as this is not the original issue here.

I'll have to leave this to better folks then me.. I think your on to something though @infograf768
I think aside from your fix, the use of toolbar::custom needs to be consistent.

@infograf768 it should not be
<button class="button-crop btn btn-primary" type="button">
but instead <button class="btn btn-primary" type="button">
the class belongs on the span.

PR #30206