I recently had a Sim donate 1,000 twice in the same save file by the same Sim (the second time happened a few days later). The particular family didn't have much money at the time, but now that they do I have not had it happen again. I thought perhaps it was because she was a young adult now and she did it when she was a teen. FYI, this Sim did not have the 'good' trait which is what I would have expected to possibly trigger such an action. I also just saw it happen in a current Let's Play, in this case the Sim was a vampire and she also did not have the good trait, so I would say it is still happening.
I use no mods or CC, play on an updated Windows 10 system, my game up to date and I have all packs but the Vampire Pack.
I agree that Sims autonomously spending $1,000 is different then them spending less then 100 or less on drinks/foods/tips etc.
Thanks and regards,
Jennifer