I have owned a TS75 for close to 4 years now and have never had the issue you are describing. Yes, if I 're-plunge' I may get a barely detectable difference. In my case I have always attributed that to me, not the saw. I am talking about a difference that _maybe_ a fingernail could catch but never enough to cause me concern for a glue-up. Never anything close to 1/8 inch, ever.
Flexing enough to cut the rail....? send it to Festool, it is under warranty and something is definitely wrong. I had never even demo'd the saw prior to purchase. I bought it, brought it home, read the factory manual and used the saw. and have never had a problem. IMHO no prior experience is required to operate this tool. With that said if it is giving you issues "out of the box", and you have consulted the manual to verify that all is in order. Then I would call the saw defective & send it to Festool. This is truly a no-brainer tool to use, especially for a professional.