Lay bets in Test Mode with a "Take SP" at-the-off persistence have been fixed in the latest Beta (great and sincere thanks!!) but Back bets with a "Take SP" at-the-off persistence are (now) going wrong.
The amount of your Back liability (ie the amount of the bet) should stay the same as however-much-you-staked when it gets matched at SP, regardless of the odds. For example if I Back bet £10.00 at umatched odds of 11 ("hoping for a nice profit of £100") it should still be a £10.00 bet if it remains unmatched at the off and becomes an SP bet at, say, SP odds of 5 (giving a profit of £40 - "not so nice a profit as I may have been hoping but hey, at least I've still only risked £10.00").
However, Test mode is now changing the size of bets if the odds are not the same as the "hoped-for" unmatched odds. It's looking at potential win amounts and adjusting the Bet amount to aim for the oringinal hoped-for-profit.
In the example of the second paragraph above, it's changing a £10.00 back bet at unmatched odds of 11 (potential profit of £100) to a £25 back bet at SP odds of 5 (potential profit still 100 but wrong as I'm now risking £25.00 - eek!)