Hope this has been resolved somehow, that’s a shame. You’re only allowed to deposit up to £20,000 a year, and a free share would contribute towards that. Dividends, however, are income from shares and do not count as a “deposit”. You’re free to earn as many dividends as you like, similar to how you’re free to make as much capital gains as you like. It’s only a deposit limit not an account limit.
the issue with the free share is that they technically have to deposit money in your account and then purchase a share with that money (hence why you sometimes end up with a free 1p left over).
so with a maxed out ISA they can’t deposit the funds to buy the share for you. Instead you could maybe use the Invest account referral so free shares go there instead?