Everything repeats in circles, ya know?
(for me it was at least 3.11)

Bluetooth PAN works with my Nexus 7 (2012) running stock Lollipop 5.1.1. However, the process of initiating the connection is unreliable. Sometimes it works on the first attempt, sometimes it works on the tenth attempt. I have not been able to spot a pattern to this behaviour. Once the connection is established, the performance is excellent, and the android apps seem unaware that wifi is switched off.

Zerox,
Are you using the Cyanogen bluetooth manager or another app to help. I have downloaded EasyTether but it is pretty sketchy wanting to add 'drivers' (a series of scripts and bins) to the N900 or other device(not sure if they have soemthing campatible with Maemo5)
http://mobile-stream.com/easytether/drivers
This looks like hard work on their part but also gives me trust issues downloading bins form random sites on the internet.
Do you often get a BT connection but not DCHP an IP address?

So it turns out my problem above is that stock Android PAN DHCP is just super unreliable, that's all(unless a Cyanogen update just fixed a problem). I give it a try AND wait for 3.5G to connect; it turns out that I eventually get a connection as long as I click "Internet Access" in the settings, verify that 3.5G is up on the N900, then check for actual access in a browser; rinse and repeat sometimes two or three tries until success.

And here we go again, planets and stars were in a rare allignment which allowed my tablet to work with PAN. No idea what dark magic was employed, but the spell has worn off.

As I type here it is working again after about 15min of repeated attempts.

No idea if it was several tries at software restarting both bluetooth radios, as well as shutting off my BT headset linked to the N900, but PAN finally worked again.

Effing unreliable android crap...

Android(Cyanogen - LineageOS) seems to have gotten better along with me learning to force my N900 onto the net usually by checking mail and then surfing from my tablet.
This BT tethering function is possibly the one which goes the furthest in extending the life of my N900 as I can leverage the connectivity with a wifi only tablet for effective web browsing.
Anyone who still has a dev account, why has this not been packaged into a deb?
Even better if some GUI scripting to let us switch it on and off form the notification area though as implimented I have it running ready at all times and love it.
Put this collection of scripts as a package on extras-testing!!

My N900 was rendered useless to me years ago from the sim card mounting issue, but it's awesome to read that we have people still using this effectively today.

You can have a look at genwall it has the same feature but more flexible.

I have set up a Maemo Garage page to preserve the files and hopefully soon upload to extras development.
https://garage.maemo.org/projects/pan900