I cannot reproduce it on my main Mac either (2018 MacBook Pro 15"). I've tried both the AMD discrete graphics card as well as the Intel integrated and both displayed the timing marks find. I'll try my other 3 Macs later this week, but I'm too busy with other stuff right now between scouts/baseball/LAX/work.
Hmmm...are there any logs, files, or information I could upload that could help figure this out? Surely, I'm not the only one having this issue, but it definitely seems to be something affecting a (very?) small number of (Mac) users.
The fact that I have this problem on all my machines lead me to believe it's a setting in my xLights or a conflict with some software.
With version 2019.24, I have a variation on this issue. If I'm back in Auto GL mode, the timing marks will show upon opening a sequence, but as soon as I zoom in, they disappear...and don't reappear again in that sequence until I restart xLights. Interestingly, I now have one sequence where the timing marks will persist.
Still showing with no problems in 1.x/2.x mode.