As reported on the German-language Wikipedia's feedback page, in this edit the VisualEditor converted
[[doi:10.1016/S0921-8009(97)00168-7]]
to
[[doi:10.1016/S0921-8009(97)00168-7|doi:10.1016/S0921-8009(97)00168-7 <span></span>]]
It is highly unlikely that this change was intentional. As it occurs within references, this issue is perhaps similar to T200971: Third-party browser add-on adds [javascript:] within references when editing using the VisualEditor and T192392: [[data:image/png;base64... added to article when editing using the visual editor due to broken browser extension-