I took a job where I was told engineers could work periodically on improving open source projects relevant to our work. In the two years I was there, I was given exactly zero time to work on such things.

I think these kinds of arrangements are dangled as a carrot in front of engineers, while the person doing the dangling knows full well that they are selling a lie. Now, such an offer is a red flag in my eyes, unless I hear a credible story firsthand from every engineer that interviews me.

Next time I am offered such a carrot, I basically will need to see it put into writing before I believe a word of it. I suspect you have learned a similar lesson from your own experience.