FreePBX 13.0.11, Asterisk 11.19.0. Mitel 6867i handsets. I had a hard button programmed for the VoiceMail Rest App. When a VM is left, the MWI lights up immediately. After deleting it through the phone app, it stays lit. Accessing VM via *97 turns it off immediately.
This means you do not have voicemail polling enabled. In the Voicemail Admin module look for option pollfreq and set it to something like 10 seconds.
pollfreq is set at 30, pollmailboxes is set to ‘yes’. MWI never turns off. I waited much longer than 30 seconds.
Resubmit the voicemail admin page.