Yes, it's a known bug:
https://github.com/koreader/koreader/issues/1010
pt isn't recommended for on-screen display but obviously the present situation is rather stupid regardless. The browser method of 1em = 12pt = 16px = 100% seems like a fairly reasonable default even if it's technically against the spec because px is a relative value while pt is absolute. I suspect it's currently undefined in order to wait for a proper solution, which is something we'd probably be able to do as well.
So the simple solution: pt_value/12*base_px
The complex solution (but possibly not that complex): use DPI to actually calculate the physical size of 12pt — however, on devices like cellphones doing so would actually result in an inferior experience… which is of course why pt isn't recommended for on-screen display in the first place. So I think the easier solution actually sounds best.
Edit: actually looking at it a bit more I think hwhw might've been mistaken
https://github.com/koreader/crengine...rend.cpp#L1876 seems to do what I suggest
In that case the issue must be something slightly different.