This isn't unusual. If you are powerful enough, the AI is willing to give you some of its planets in exchange for peace. If you are exceptionally powerful, it may even give you ALL of its planets. I consider the last part an unconditional surrender of the AI, but it still feels weird.

this is something that is routinely done when playing for score. qrtxian there are some conditions that are weakening the swindle tactic, most of them you'll see in the diplomatic screen. such as "they don't trust as" if you've attacked them previously after having had a peace and so on.

quicksaving and reloading might also help

the way such a game is started always on suicidal + fastest techrate, takes like ~~~30-40 turns to pull off (declare war + offer peace for all planets). by that time most if not all AI planets will show red numbers (or at least, not very high income so he will give them away)

I remember one game in DL where I had to relocate the last resort planet of the AI several times (by declaring war, offering peace + adding the new planet) but when I wanted to end that game this way he refused. had to invade traditionally this time^^. 

Since you bring up playstyle: do you generally play smaller galaxies? My experience comes mostly from the sort of settings where not getting double digits in the colony rush means you're in trouble. If you're talking about a small galaxy where the average empire size is only 4-5 planets, then yeah, swindling away an "entire empire" would seem much more in line with what I've seen.

one offtopic question

do you know what precisely triggers this diplomatic penalty "- your tendency to conquer other" ?

is it when you completely annihilate an enemy or if you generally conquer worlds with troop transports?

In one thread a few years ago, I believe the answer was taking the last planet.

One player advocated leaving the enemy one low pop planet in the midst of the others so that it would flip to you by influence.  Of course, that does not really work in DA if one has built the MCC.

Nobody forces you to ask for planets... If you find a particular tactic to be a cheesy exploit, simply don't use it!

I only accept planets when negotiating when I feel it's a realistic and reasonable demand.

  Its rare in my DA metaverse games to be in the planet taking/retaking situtation.  Usually the AI have colonized the region with less than maximum initial population - therefore their nearby developed planets are also low on population;  their shortage of troops will hinder their retake invasions.

  Demolish all the research buildings and factories which are not on bonus tiles if the defecit will be more than a couple of months.  I do this when a race has surrendered to me and I am low on BCs.

    Quicker than building influence starbases.

  You might have caught them at a bad time (sudden military weakness / too busy elsewhere / lost election / loss of resource starbases / and that sort of thing).  Plenty of stats to sift through - I think you would find a non-glitch reason somewhere.