Call to L.markerClusterGroup affecting other groups added to map

[Noxdor]

• [x] I'm reporting a bug, not asking for help
• [x] I'm sure this is a Leaflet.MarkerCluster code issue, not an issue with my own code nor with the framework I'm using (Cordova, Ionic, Angular, React…)
• [x] I've searched through the issues to make sure it's not yet reported

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How to reproduce

• Leaflet version I'm using: ^1.7.11
• Leaflet.MarkerCluster version I'm using: ^1.5.3
• Browser (with version) I'm using: Tested with Edge and Chrome
• OS/Platform (with version) I'm using: Tested with Linux, MS WIN
• Create a FeatureGroup with a single DivIcon inside and add it to the map
• Create a MarkerClusterGroup, add some DivIcons and add the group to them map
• See the FeatureGroup's DivIcon slide around on the map while zooming, even though it is not part of the MarkerClusterGroup

What behaviour I'm expecting and which behaviour I'm seeing

I am expecting the two groups to be isolated from each other. The effect of the MarkerClusterGroup should not be applied to the FeatureGroup. But it is, and we tested it and it happens as soon as we call L.markerClusterGroup(). If we don't call this function, the FeatureGroup's marker doesn't slide on the map.

Minimal example reproducing the issue

• [x] this example is as simple as possible
• [x] this example does not rely on any third party code

Using http://leafletjs.com/edit.html or any other jsfiddle-like site.