This isn’t actually a bug. If you want to change the high/low watermarks by using a percentage you must end the number with ‘%’.
The correct command is below.
/opt/couchbase/bin/cbepctl localhost:11210 -b default set flush_param mem_log_wat 90%
/opt/couchbase/bin/cbepctl localhost:11210 -b default set flush_param mem_high_wat 94%