When invoking Replace multiple times, its options (Match Case etc.) are remembered between invocations. But when invoking Find in between, the Replace options are reset to the values of the Find options. That's not the case the other way round: Chaning the Replace options doesn't affect the Find options.
The behavior is as if two different sets of Options are stored for Find and Replace, but invoking Find saves the Find options to both sets, while Replace only saves the Replace options to the Replace set.
Either Find and Replace share options, then they should be copied in either direction (or both should use the same saved set of options), or they don't share options, then invoking Find shouldn't affect the Replace options.