I would ask at immigration as you prepare to cross the border. But, if you bought multiple re-entry then I would think it would be fine as long as you haven't exceded the number of exits.
When I flew here in 2022 I had to book an onward before Japan airlines would let me board for my flight to Thailand. The onward doesn't have to be really done. And it can be a any mode of transport that shows you planning to leave Thailand. But, once you get to Thailand you can always change your plans. There are even companies that you pay a small fee and they book you a flight that won't be used. I wish I could remember the name of the company but it has been a couple of years since my kids told me about it.
Because there are a lot of ignorant people in this group. They feel better about their pathetic lives when they put someone down for asking a question. Typical bullies that peaked in highschool and came to Thailand because uneducated Thai girls will take their money.