This also happened with us, two members lost their clan xp overnight.
Runeclan shows them leaving and joining but in game their join dates dont reflect that.
runeclan shows a player named "null" joining the clan at the same time as the incident happened.
This issue was picked up by our discord bot 13.11.2019 and only 2 people were involved, and one of them has gained clan xp since then.
@The Pandaa, after checking your runeclan, you also probably had 2 people effected, Geatry and Walencia
15-Nov-2019 15:26:53