Nextcloud is somehow different from other apps in this regard, as no other light (white) apps on the same device have this issue. For instance, here’s Play Store on my affected device (running Android 6.0.1):
Also, I seem to recall this only having started to affect Nextcloud quite recently, perhaps sometime in the past month or so.
Switching to dark mode is a workaround on my device at least (there the Android buttons are light-on-black). I can also confirm that the issue doesn’t affect my newer devices with newer Android.