This comes up as a question time and again…
My educated guess is that in this instance, this is just you placing a buy order at the best market price using the “instant order” and the Market Maker(s) (exchange market liquidity providers) just taking your oder at the top end of their buy/sell range at the time of order.
I would also note, having just checked the 1 minute chart for the ETF that you purchased, it doesn’t look very liquid which could widen the bid/ask spread a bit
As an example (made up),
price shown (mid of range) £30.55
buy price (market) £30.60 (price you pay to buy)
sell price (market) £30.50 (price you get paid when you sell)
MM spread £0.10
As I mentioned, the spread can vary due to the amount of orders being place and general liquidity for the item that you are trying to buy or sell.
If this is what I imagine that it is, it’s totally normal and it’s how the markets work. This has nothing to do with Freetrade and if you want to avoid this kind of thing then I recommend that you move to using “limit orders”, rather than “instant orders”. However, if you offer to buy something with a limit order at the displayed "mid’ price, you may find that the MM’s just ignore your order as they want to sell at the top end of their range and buy at the bottom of their range. So, it’s up to how you place the order, but it doesn’t change whether the MM’s will take your order at that price.
All that I would add, don’t really worry about this as you quickly get used to it and as long as you buy something that moves in the right direction, this bid/ask spread quickly gets absorbed within the asset price movement.
Best of luck and it’s sounds like you’re fine.