There was already this answer regarding the basic rules of rendaku. However, it seems that these rules do not apply at all to proper nouns, especially toponyms. Examples include 秋葉原｛あきはばら｝ (no rendaku) vs 千葉｛ちば｝ (rendaku), 川崎｛かわさき｝(no rendaku) vs 宮崎｛みやざき｝ (rendaku), and 石川｛いしかわ｝ (no rendaku) vs 神奈川｛かながわ｝ (rendaku), etc. I find no patterns in those pairs. 秋葉原｛あきはばら｝ is especially confusing, since it completely reverses the general rendaku rules, and I'd expect it to be pronounced as あきばはら instead of あきはばら.
Therefore, is it the case that 連濁 in proper nouns (basically toponyms and surnames) are pretty much random and doesn't really follow any pattern? Or are there special rendaku rules for proper nouns?
By the way, it would be also welcome if anyone could explain why 秋葉原 in particular has this eccentric voicing pattern.