Disallowed advertising costs?

A little confused with this article

https://www.thetimes.co.uk/article/rising-storm-damages-put-the-wind-up-...

Apparently some advertising costs are were not tax deductible and Hiscox are going to do a prior year adjustment. On the article it says it was identified internally and not by their auditors or any tax authority.

I'm curious what marketing expenses (relating to advertising) is not deemed to be tax allowable?

They prepare accounts to IFRS.