Slovak isn’t maintained in LanguageTool, so I cannot comment on that example. A typical reason is that a rule raises too many false alarms. You can only turn on this rule by turning it on in the sk/grammar.xml and restarting LT, i.e. by removing default="off".
Thanks for the idea. I fixed it by login into Docker container as root and edit ./org/languagetools/rules/sk/grammar.xml and here remove default=“off” from rules I want to enable.