The consonant /r/, whether original or from earlier /z/ through rhotacization, was generally not affected; it occasionally shows gemination in Old High German, but inconsistently and this may be an analogical change.
[1] In the early history of most individual Germanic languages, syllabic /ij/ was generally lost, while consonantal /j/ was retained.
It therefore appears that Sievers' law was still productive at this stage, and adapted to the new syllable length by changing the suffix from its consonantal to its syllabic variety.
West Germanic gemination also operated inconsistently on consonants followed by /l/ or /r/, e.g. Old English æppel 'apple' < Proto-Germanic *aplaz.
By historical times (c. 800-900 AD), all of the West Germanic languages except Old Saxon had lost medial syllabic /ij/, but not before any /j/ that it may have developed from had triggered i-mutation.