I think this is a bug, since the option you need is available if exporting the sidecar through the Media Export option.
You are probably exporting using the three dots in the Text (Caption) Panel. When doing that, there are no settings for the export. Note that this option for export does not appear to be documented (yet).
Instead, export using Ctrl-M (or File -> Export -> Media), choose "Create Sidecar File" in the Captions tab, pick Subrip, and finally, do not check "include SRT Styling." Unfortunately, to use this option, you must also export either Audio or Video (or both).
The checkbox should be available anytime srt is exported.