FTP is not readily calculated off of 20 minute power, and especially not off of 20 min NP power, or NP period. NP can be fairly easily busted and skew results.
Secondly, every ride should not be an effort to try and improve on a previous effort.
Thirdly, indoors sucks the life out of most people, and it's a rare individual indeed that can have similar power numbers indoor and outdoor.
Fourthly, doing group rides in which you bite off more than you can chew and keep coming back for more can be a huge boon. I'd keep doing those. And they're hopefully fun.
Lastly, many of the changes you're seeing are small enough to be attributed to the error percentage range of your power meter. Meaning I wouldn't fret too much about any pluses or minuses, especially in the shorter term. You could probably go out for a big meal and get up tomorrow morning and pop off some new high numbers if you wanted.