Originally Posted by wanderer9020

at SEA, your documents will be check for your final destination DPS ( covid test, visa , etc). Two boarding pass will be issued. The airline cannot control what you gonna do while waiting for your 18 hours in transit. As long as you have the right to enter Singapore, you can enter as you wish. And since you have already your boarding pass, no documents check will be done.

The OP is a different story, I read at he has separate tickets, therefore, covid reqmtns for Singapore and another covid reqmts for his next destination will be check according to the testing time frame reqmts of both countries.