Why there is an E after the U in Robt Muellers name

[jimmy the one]

Robert Muller's proper name in German was spelled with an umlaut over the U, but only one E.
This might be Muller with an umlaut if my copy and paste transfers the umlaut: Robert Müller.
Robert Muller of course died yday. RIP Herr Muller.

In German, an umlaut (oom lowt) is a diacritic which slightly alters the pronunciation of the 3 letters a, o, and u, as well as the couplet au. An umlaut is two dots atop the letter a, o, or u - one more dot than the similar dot above the English lower case I. The colon : would be an umlaut sideways, rotate 90 degrees for the umlaut.
The umlaut alters pronunciation by simply rounding the lips, like kissing mum or sister on the forehead, and then pronouncing the letters in English as saying the long a, long o, long u (pronounce the U as oo, not yoo). Do it now and you can detect a slight difference between the long English vowel and the umlauted vowel, though it sounds nearly the same with a tonality difference.
The umlauted German word Madchen (young girl) is thus close to Maid chen, rather than mad chen.
German Ostlich (easterly) rhymes close with toast lick. OL (oil) close with sole. Muller as moo ler.
The umlauted au is most famous in the German 'fraulein' where the umlauted au sound is like English ow, so close to Frow line - not froy line as some do. Fraulein is also a young lady.

My iPad does not have umlaut capability, which was essentially the reason for the added E after the 3 vowels a, o, and u, as I will explain.
About the 1960's, perhaps earlier, American and British typewriters and keyboards also did not have the capability to type an umlaut. So when umlauted words and names such as Muller appeared in English newspapers and many documents, the umlaut was perforce missing, German words mispronounced, and Muller was pronounced as a short U, rhyming with duller.
A few umlauted words such as Uber were pronounced more properly as oober, surely due the well known song 'Deutschland uber alles'.
The Germans became so perplexed at hearing their umlauted words being mispronounced so frequently as short English vowels that they developed a strategy to correct the misperceptions.
They realized that by placing an E after the A, O, and U, of course easily done on typewriters, the English speaker would pronounce the 3 vowels as long rather than short, which is indeed a rule of thumb in English, though not absolute. Due, rue, sue, doe, hoe, foe, toe, aeleron, aerobics.
And thus the written German umlauted word would be pronounced more properly as a long vowel. That was the hope and desire!
Alas, over the years the E stratagem has tended to be disregarded at least in America, as evidenced by our general mispronouncing Muller, still as rhyming with duller.