Words in vocabulary definitions that use umlauts (ü,ä,ö) are split up based on them when clicking on the words to look them up in the dictionary. For example, the word "Hügel" in the German definition for 丘 is split into "H"and "gel" when clicking on it.
I encountered this issue in the Android app and Firefox.
To reproduce:
1. Set vocabulary language to a language that uses umlauts (e.g. German)
2. Look up a word that contains an umlaut in its definition (e.g. 丘) and click somewhere on the definition.
3. The dictionary looks up only a part of the word you clicked on.
In the example below, I would have expected it to look up "Hügel", but it only looked up "gel".