Keyboard Dictation Doesn't Work With Powerbeats, But Siri and Voice Memos Work Fine

This worked fine for me for years on my iPhone 11 Pro Max and stopped working around 3-4 months ago. Not sure if it was after an iOS upgrade or not, but both my iPhones have been updated since the problem began. It has never worked on my iPhone 14 Pro Max.

I have two pairs of Powerbeats (MWNV2LL/A) and have duplicated the problem with both pairs of Powebeats on both my iPhone 14 Pro Max (currently iOS 16.3.1) and iPhone 11 Pro Max (currently iOS 15.6.1). Here's what I know:

• With both pairs of Powerbeats on both iPhones:
  • Enable Dictation is on in keyboard settings for both iPhones; turning it off and back on does not resolve problem
  • Both Siri and Voice Memos work through the Powerbeats microphone
    • Confirmed audio is coming through the headset microphone (fingernail on microphone opening shows sound detected, and that sound is recorded for Voice Memos)
    • Voice dictation on keyboard (US English) does not work; it's as if the microphone is muted, nothing is detected at all
• Other Bluetooth headsets work normally on both iPhones
• The iPhone microphone (without headset connected) works fine on both iPhones
• Complete erase and "setup as new iPhone" for my iPhone 11 Pro Max resulted in the exact same problem

Any thoughts on what might be going on or how to fix it?

Thanks!