Hey there,
I'm running iOS 27.2 on an iPhone 17 Pro. Sometime last week or thereabouts I noticed that reading in the Kindle app, certain words were being mispronounced. I was using the Samantha voice. After some investigation, I realized that it was only the letter 'f' that was being mispronounced. Then I went deeper and found it was only single, lower-case 'f's that were messed up. So "five" sounded like "ive", "scientific" sounded like "scientiic", but "Frankenstein" or "sufficient" were both OK. Needless to say, enough English words contain single 'f's that this made reading very unpleasant.
To my surprise, hitting play on the same text in VoiceDream Reader had the same problem. The voicedream text was an epub from bookshare; the kindle text was checked out from libby.
I changed the voice to Nicky, and to Reed, and the problem persisted.
Then I changed the voice to Alex and it was fine!
Also, I just thought to try a different text purchased from the Kindle store vs. checked out from Libby and that's fine!
Any wisdom from this lovely community would be much appreciated.