the reality is there is no new rules, what there is is published guidance for immigration officers. That guidance says you get 1 extension on a 60 day visa exempt entry and if you have a 2nd visa exempt entry shortly thereafter you will get a 7 day extension (in reality that's not an extension, it is an extension denied stamp which gives you 7 days to leave Thailand)
If you spend 90 consecutive days inside Thailand you are legally obliged to make a 90 day report. The first report after each entry must be done in person at your local immigration office
In theory you should not have a problem but you should not have been in Thailand when the visa was issued, you should have remained in Cambodia, so not sure if they will have something to say about that.