Hi Tom
Thanks for looking into this and providing a very clear and detailed response.
Now, this makes sense to me. It’s more a design feature than what I wrongly assumed was a bug. It’s a shame that I wasn’t just told that earlier 
Anyway, in the meantime, a suggestion for this could be that before the payment is defaulted to an instalment plan (when the user has pay-in-full selected), it prompts the user to either choose the change or reject it and remain pay-in-full. If no response within a certain timeframe, it places the payment on an instalment plan anyway.
However, I now think there is another nuance/complication to this.
My payment date is 25th of the month and I keep track of my finances with Account Tracker (smartphone app).
I have had my 25 April payment in the app (£576.74) since the 14-day mark on 11 April. On 22 April, I was transferring some money between accounts - the Flex payment on the front screen for 25 April was still showing as the amount above. However, when I looked at my app yesterday (23 April) I noticed it had gone up slightly by £0.55 (only a small amount, but still, I wouldn’t expect a payment to go UP during the 14-day period between “statement” date and payment date).
Customer Service have been unable to provide an explanation at this point, but I think I’ve worked it out for myself.
I went through some of the transactions and noticed that a Lime pay-as-you-go payment from 7 April was on a 2-month instalment plan rather than pay-in-full. I had remembered this transaction from earlier in the month (£2.45), it had defaulted itself to an instalment plan rather than pay-in-full (as per the process you have outlined in your message) and I had changed it back to pay-in-full.
I used a Lime bike on 21 April for £2.74 (and it had now appeared on this same transaction (presumably using the same auth code) from a full 2 weeks prior(!) and increased the payment amount to £5.19. It had changed it back to a 2-month instalment plan and as a result, because the original authorisation code was prior to the 11 April (my 14-day cutoff period for 25 April payment), it therefore increased my payment amount 2 days prior to the payment being due even though the transaction (if it had it’s own new unique auth code) should be paid on my next payment date in May.
The increase in £0.55 was the increase of the payment due to the plan. If I edit the transaction to just a 1-month payment, it increases my credit card payment by that amount (£2.74)
I suspect this is another complication of the Flex/instalment system.
Anyway, thanks again for the explanation - I really think there would be a benefit to prompting the user.
“This transaction amount has been updated by the retailer. We will change this to a 2/3 month Flex instalment plan in 2 days. If you do not want this to happen, please edit the plan and change it to pay-in-full”
… or words to that effect…