Good morning Abby,
As far as I’m aware, this is not a known issue. I tested a quick sample just now in IE11 version 11.285.17134.0, publishing to HTML and the captions appeared to work as expected. Same expected behavior with run mode and publish preview as you both mentioned.
I’d like to take a look and debug with the titles that you or Deb are experiencing the problem with. That should help me track down the issue much more quickly. If you are able to provide me the export pkg or name of your title and if possible the video in question, that would be helpful. If you prefer to just give me the name of the title and your org, I can look it up through that also.
Hopefully can get this turned around for you guys quickly and apologies for the inconvenience