Kodi does not resume payback correctly for STRM files.
I’d like to suggest to incude this patch in the next version for OSMC to address the issue
issue: kodi fails to resume .strm files · Issue #19981 · xbmc/xbmc · GitHub
pr: Resume/show bookmark for strm files by Acidzero2020 · Pull Request #16076 · xbmc/xbmc · GitHub
It’s been discussed on the Kodi forum and there still seem to be issues: Resuming STRM Files
This might also be the reason why it’s not been merged upstream. I guess, we should wait for Kodi to merge this. 
yeah, I read that forum discussion, it’s where I found the patch I proposed in my post.
the issues reported in that forum are related more to the addons workaround proposed, not to the kodi patch itself (at least as I understand it).
Maybe it would be possible to add it as an option, disabled by default, in the Settings…
I’ve posted this as a question in the corresponding Git PR… We should wait for Kodi to implement this.