Yes, it's most likely a coincidence. We can see this by comparing several titles:
The formal さま, which is clearly Japanese, was shortened to さん, which is now the most common and general title, and is more or less unmarked. さん was further reduced to the hypocoristic ちゃん, which is also very common, though not quite as much; and there is ...
German and Japanese similarities here are purely coincidental. Japanese -chan derives as likely baby-talk from regular suffix -san. Similarly, we have regular -sama (which itself is the source of -san) and baby-ish -chama. See also most any JA-JA dead-tree dictionary, or the Daijirin entry here (see the third entry down), clearly stating:
I am not a native, but I would make two guesses as to why it sounds funny:
1) It could be because ちゃん expresses that the speaker finds that person endearing. Since お侍 is a position that holds superiority, the use of ちゃん now becomes condescending and rude.
2) The other reason I can think of, is that in japanese the combination of お and ちゃん that I have seen ...