What OS and Lightroom version are you using. I just did a "Print to File" of about a 100 RAW files in the Print module and then went back and did some editing on other photos in Develop and the task completed without error.
MacOS 10.13.4 and Lr 7.3.1
I am creating PDF contact sheets. If i leave the print module before the file begins to actually render, it will stay in "preparing to print" mode. Its just annoying as if i am batch exporting 40 contact sheets, it slows me way down as i have to wait to each one to start rendering before i can get the next one going.
I just tried on contact sheets and got the same successful results as before. I guess we will have to wait as see if someone else is having the same issue you are.