iOS Mail: Non-ASCII characters garbled in subject header when followed by whitespace

I noticed recently that certain German characters (ß, Ü, Ä/ä, Ö/ö) are displayed incorrectly in the subject header of messages if (and only if) they are followed either by a space or no character (within the subject header). This only affects Mobile Mail on iOS, but not Mail on macOS. Even more mysteriously, the lowercase Umlaut ü is not affected. The example subject lines L(ü|ä|ö) are displayed correctly in macOS Mail, a webmail interface and other clients (like mutt). The message source looks as expected:

Subject: =?iso-8859-1?Q?L=FC?=
Subject: =?iso-8859-1?Q?L=E4?=
Subject: =?iso-8859-1?Q?L=F6?=

In Mobile Mail, the subject looks like this (as I said, lowercase ü is the only umlaut not affected): Lü L� L� Does anyone know about this bug in iOS Mail? Thanks, Jan