Mysta/Kuro's British isn't he? In the UK the company does taxes first, not the employees. Only time you'd really be expected to do your own taxes is if you were self employed. With him having been working for Niji I'd assume he assumed they would either do it or at least provide an accountant.
Whether he hired an incompetent accountant or Niji did, it's not clear unless he volunteers more details, it's still a pretty bad look to have one of your most popular talents getting into massive financial problems and then being told "not our problem" by their employer even as they continue to earn off him. Especially when it's the head of a rival company who ends up spending 4 months working through his issues instead and ends up poaching the talent as a result.