Unfortunately you’re right - the _t cookie still doesn’t have a path associated. It’s something we’d like to do, but ‘subfolder’ installations are fairly rare, and multiple subfolder installations on the same domain are even more rare. So it just hasn’t been prioritised.
I took a look at one collection of subfolder sites that we host, and it looks like our workaround has been to rename the cookie for each site on the domain. So, assuming you’re using our standard install YAML files, you would add something like this to each one:
env:
DISCOURSE_TOKEN_COOKIE: _t_mysite
(where mysite is different for each forum)