I don't know about standard behaviour, some people consider behaviour normal that I consider slimy.

I heard at the time that a WP Engine employee had told someone that his/her pledged "full time assignment" had shrunk to a small number of hours per week after a while. That's hearsay, I heard it from someone who doesn't work at either of the two companies and it must be third-hand. But if that were the case, and if the pledge didn't say how long the full-time assignment would be kept up, some people would say that the promise was kept. Those people probably also would say that you shouldn't trust a handshake agreement, and please have a lawyer check the written agreement, and if you run an open source project, copy the policies Oracle has for JDK.

What I say is that there's a conflict waiting to happen, if one of the parties expects the pledge to be kept up for a significant period of time and the other expects to tiptoe away shortly later.