Hi @Klaas Tiktak
I suggest you go to Developer tag > Shape Design group, protect your shaps to have a check.
Besides, it's recommend to let the connector glue to the shape, please go to Home tab > Editing group > Layers > Layers Propertities, make sure the Glue box is enabled.
Then go to View tab > Visual Aids group > Snap & Glue > make sure the Glue box is also enabled.
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