@Tom ******
again that's up to the discretion of your immigration office. I have seen reports of people successfully supplying documents from two banks, being in a similar situation as you.
Edit: and by similar I mean their account dipped too low, for whatever reason, but they could provide evidence that they have over 400K in Thai banks at all times.
Other reports say their immigration accepted that it was just for a few minutes/hours and at closing time of the day the amount was correct.
Best advice anyone could give you is ask your local immigration. Everything could be just fine, or they will tell you your options. One could be the 60 day "visit family" extension.