@adrw Do you mean the filter UI itself is missing or just the filters are missing? One thing you can check is if the filters are there on the server. You can do this by:
cd /home/yellowtent/boxdata/mail/vmail
find . -type f "*.sieve"
To add to what @jimcavoli said, I think the reason is that the sieve language is probably hard to parse. Curiously, rainloop has a whole meta language going with comments in the sieve file which helps it parse.