Hi JollyRoger, thank you for your post.
The 12.5% figure came from a journalist rather than from the club/Broughton directly.
Based on direct quotes from Greg Broughton https://www.lancashiretelegraph.co.uk/sport/23788076.blackburn-rovers-potential-david-raya-windfall-revealed/
Rovers are due "around £5M over the next few years" if/when Raya's transfer to Arsenal completes for £27M. This would mean that his sell on fee is somewhere between 20% and 25% of any profit (Broughton did not give exact details so we had to make an estimate) As such, the figure set in game is as correct as can be deducted given the available information.
In real life there may be multiple (far more nuanced) clauses (that are not available for us to set in game) which give the total £5M (ish) fee a different makeup, but as far as I am aware, in game, the only way that Rovers can earn any further money from Rayas transfer from his transfer from Brentford to Arsenal is as part of a one off, sell on fee, which is what has been set.
The clauses showing twice is a bug which if being investigated and will be removed as soon as is practicable.