It's a known issue
Quick fixes to try (in order). As an aside, personally I recommend upgrading the digital assistant to use Gemini (it's much better) rather than the 'stock' one that comes packaged with Android OS if you've not already done (it's in the Google Play Store). I've an S25+ with Gemini and the voice nav still works, plus it's just generally much more powerful than the stock Digital Assistant, and its free.
Retraining/re-enable voice model (most common fix):
Google app → Profile → Settings → Google Assistant → Hey Google & Voice Match → Retraining (say phrases 3x). Toggle Hey Google off/on.
Clear Google/Maps cache/data:
Settings → Apps → Google → Storage → Clear cache (then Clear data if needed, re-login). Repeat for Google Maps and Assistant apps. Restart phone.
Check/disable Gemini interference:
Settings → Apps → Choose default apps → Digital assistant → Set back to Google (not Gemini/Samsung). One UI 7 beta users report Gemini hijacks voice commands.
Maps-specific voice settings:
Google Maps → Profile → Settings → Navigation → Play voice over Bluetooth (on) + Voice playback language matches your phone locale.
Microphone/permission reset:
Settings → Apps → Google/Maps → Permissions → Microphone (allow all time) + Phone (allow). Test in safe mode (hold power → long press Restart).