I can do things like this: (bossbar related)
set title of bossbar {BossBar::%{_uuid}%} to "Test"
or
set progress of bossbar {BossBar::%{_uuid}%} to {_hpload}
but I get a error if I do: (adding/removing to bossbars)
add player to bossbar {BossBar::%{_uuid}%}
or
remove player from bossbar {BossBar::%{_uuid}%}
Using:
1.19.2 Paper
2.6.4 Skript
2.0.9 Skellett
Only other plugins that I think could effect this would be Skbee but bossbars are disabled in skbee
ExtraSmartMiner
commented
1 year ago 
I can do things like this: (bossbar related) set title of bossbar {BossBar::%{_uuid}%} to "Test" or set progress of bossbar {BossBar::%{_uuid}%} to {_hpload} but I get a error if I do: (adding/removing to bossbars) add player to bossbar {BossBar::%{_uuid}%} or remove player from bossbar {BossBar::%{_uuid}%}
Using: 1.19.2 Paper 2.6.4 Skript 2.0.9 Skellett
Only other plugins that I think could effect this would be Skbee but bossbars are disabled in skbee