Unfortunately the issue has since returned, even with unrestricted battery mode for WhatsApp. As a side note, I am seing this behaviour using battery saver mode, however, with around 60% of battery remaining.
What is curious however, when I tried this last time and the playback stopped when in another app, I just turned off the screen and let my phone rest and doing so after about 5 minutes, the voice message just resumed playback. Don’t know what to make of that but maybe that helps