Work permit and visa goes together. I've just been through the almost the same thing when returning from work trip. Work permit expired while I was abroad. Visa still valid. I got stamped into the country based on visa. When company tried to resolve the work permit, they were charged for overstay from date work permit expired, and I was given 48hours to leave and come back on tourist visa and restart entire process.
10 years ago, my first time to Thailand, I also had to show 20,000THB is cash. Few years later, they actually take a copy of the money serial numbers because people in groups used the same 20,000THB.