Definitely not anything from 20 years ago (though I think it is very safe to say that there are +20 year old bugs in tables that are all on Apple’s side!). What you are describing here is a newer problem that has cropped up in the most recent updates—I have merged your report with the main bug thread on it.
It appears that something relates either to the macOS 26 release, or perhaps fixes we applied to the software in reaction to the great swell of bugs that updated foisted upon us.
Sadly to date we have no clear reproduction on it. It seems to impact particular projects repeatedly, but once the conditions are attempted in a test project, they either don’t show up for us, or not even for the person trying to make a sample project.
As this is happening in your template, in projects created straight out of it with no modification, it might very well be something we can see as well then. I tried to access the template, but it seems Tumblr doesn’t allow one to scroll without making an account (?!), so maybe if you could send it as an attachment in a PM to me, I would be grateful!