Month names collection / metastudy

[Koen G]

An interesting one: 
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Hans Zukunft, Das goldene Jahr (from your Minnereden Band 2 (MR2))
1450AD, Swabia

jonius
julius
Jonius for june 

Strange, I went to double check and it clearly says "jonius" indeed.

Across the French and German examples, I count about 8 instances where we get "o" in June but not in July. While the phenomenon overall can only be described as very rare, I feel like there should be a reason why June is more susceptible. 

In Latin, the difference between Iūnius and Iūlius is just the middle consonant, n/l. Was Iulius more anchored because of Caesar? I'd imagine that association is lost soon enough, but maybe not when one is used to the vernacular names and still sees June and July as Latin words?