Did the fabric lose noticable/significant amount of indigo after that first hot water soak?
As a general rule any decent indigo rope-dyed fabric will not lose any significant colour with an initial wash. Indigo is oxidised onto the surface of the cotton and is only removed by abrasion (wear). Any indigo you do lose will be loose unoxidised indigo and would rub off very quickly with wear anyway.
And as
@Kasi suggests, drop the crew an email at shop@ironheart.co.uk, you won't find better-qualified people to answer questions like this, it's their job.