When you copy into protonmail, being a web client, it looks like it’s using the plain text version. Note the lack of formatting differentiation, though it’s hard to tell since you’re overriding fonts.
Contrast with pasting into Thunderbird:
It’s picking up on some of the formatting, but not all.
If I copy, it picks up everything:
I guess to figure this out, you have to differentiate whether:
- not all styles are copying
- not all styles are pasting
Starting from a clean browser with no extensions might help figure it out? e.g. if I use a private browser with no extensions etc. I get:
this script I wrote will also give you visibility into the clipboard, which may help.
(or maybe Arch » Fedora 
)