Interesting, I have the same problem, so it’s not just me.
https://discuss.ai.google.dev/t/2-5-flash-audio-native-output-broken-in-de/89973
For me it feels quite random, cause it can happen in the 2nd answer in the 8th or whenever, but one thing you mentioned seems interesting to that it happens more offen when interrupted. For me it was actually changing in frequency based on the topic, but maybe just coincidence.