Made some further discoveries. I tried playing a round of golf with Michael and Trevor again, like before, and it seemed to work fine...until Michael's ball disappeared on the second hole, causing him to swing his putter indefinitely.
The only way to get out of that was to kill him. I had a bit of fun and dropped a golf cart on him. Ran off, escaped the cops, then, when trying to hang out with both of them again, the blips disappeared AGAIN! That golf minigame is mega-bugged, you're almost better off ignoring it altogether.
Anyway, I tried my proposed solution mentioned in the last post, but it didn't work. The blips still didn't appear, even when hanging out with Lamar.
Interestingly, I was able to visit the strip club with Michael and Trevor, and they were able to watch at the pole (only Michael watched the pole last time I tried this, and Trevor wasn't interactable - they both were interactable this time). I thought maybe the blips would appear after dropping them off afterwards. Nope.
saved on my phone
exited the whole game
went back to the golf course
paid the $100 to "play" another round of golf (I could actually play this time!)
dropped Franklin with Spooner, like last time, after his walk-in animation played
6.5. (paid myself $5100 back for that BS )
phoned up Lamar (blips are back!)
let him die in a gunfight
phoned up Michael and Trevor (blips are back!)