I'm 99.9% sure that there is a ~32K size limitation for GetHelpContentData.addHelpPage(), more precisely: 32413 bytes work, 32414 bytes don't. The external file is read to a string, both via Script.loadHelpFile() and manually, and added via addHelpPage(). Both OSP and non-OSP scripts have this limitation and page simplicity/complexity does not play a role.
Is this an intentional limit for user scripts only? It's not something that needs to be increased, I could work around it, but it is surprising because there are significantly larger pages in standard help. This doesn't look like a bug, asking purely out of curiosity.
But this one is most likely a bug. Even when both OSPs are cropped to under 32k, if both are loaded and activated, clicking help for either one makes the help page hang, a "loading" icon shows up but proceeds no further. There are no errors in browser console and all file names & titles are unique. Enabling only one makes the help pages work again.