Hmm very odd, nothing related should have changed between 5.70 and 5.77. If you can switch versions and find a difference, perhaps you can narrow down which version between them actually causes the change (5.75 and 5.76 are the two versions between 5.70 and 5.77)?
You can set REAPER to not allow projects to override the samplerate (in the audio device settings), which should fix the issue for you.
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