the "oa"-sound in German before r

It seems to be that a lot of speakers tend to make a rhotic vowel or
something
before r while others do not and I'm not sure if this is pronounciation
can be called
standard or not or if there's simply no standard about it.

here are some samples :
Koran [ko: ra:n] [ko@ ra:n]
Meran [me: ra:n] [me@ ra:n]
Admira [at mi: ra: ] [at mi@ ra:]

the rhotic vowels seem to dominate in German words

Beeren [be:@n] (most common) [be:@ ren] (common) [be: ren] (uncommon)

.