Testing both in this forum, and at this Markdown test site, it’s reproducible. So it seems it’s just a feature/quirk/bug/limitation of Markdown in general.
This is a test of no spaces:
Thisisatestofnospaces (works)
Thisis**(atestofno)**spaces (broken formatting)
Thisis**(atestofno)**spaces (testing if escape characters help. No.)
Workaround: Put the brackets outside of the bold formatting. Not a “fix”, but it works.
Thisis(atestofno)spaces
今天的气温是(三十度)
Thisis**atestofno**spaces
Thisis**(atestofno)**spaces
Thisis**\(atestofno\)**spaces
Thisis(**atestofno**)spaces
今天的气温是(**三十度**)