I don’t believe they do this. Correlation does not equal causation.
Everything Freetrade said here probably still holds true:
In particular, “When prices are moving rapidly, particularly in one direction, market makers may find it difficult to provide quotes because of the risk they incur losses by being left with a position to fill, even though the market price is moving against them.”
I had trouble buying Shell at one point last year for precisely this reason, and I could see the large changes in price every time pricing data updated in the app. When I finally did get a successful order through, the price was significantly lower again than it had been show when I placed the order.
tl;dr, market volatility creates difficult trading situations at times, not Freetrade playing about with secret measures. I mean, why would they say “You may also find that the execution price you receive varies more than normal than what you saw on the stock screen, as a result of prices moving very quickly.” if they had a feature that blocked orders when the prices move quickly?