Good to find this post as I've noticed the same thing and wondered if I was going crazy.
For example if I lay £10.00 at odds of 3 with "take SP" persistence, my potential loss is £20.00. If the SP odds end up as 5, my lay bet should auto-adjust (as it does on real Betfair) to a £5 lay so that my potential loss is still £20. However on test mode the lay bet remains at £10 (incorrect) giving a potential loss of £40 if SP is 5.
Is there any news on a fix for this as it's making test mode lay bets (with sp persistence) inaccurate?