A week of disrupted sleep, more alcohol than usual, and meals that look nothing like what you eat at home will show up on your skin within a few days. Your skin reflects what is happening internally, and travel pushes all of those factors in the wrong direction at once.
Most people attribute it to jet lag or the change in water, but diet and sleep are doing more work than any product in your bag.
Keeping even one good habit going throughout a trip makes a real difference. Getting close to seven hours most nights, or keeping one meal a day close to what you normally eat, gives your skin something to work with.
It is genuinely what separates people who come back from trips looking good from those who spend the first week home recovering.