I agree leeanne86, this is very annoying. Until Apple fixes the timer bug I simply speak to the watch "Hey Siri, set a timer for 5 minutes" (or whatever many minutes I want). After the time is over, the "repeat" button still works (if you want to repeat the timer count). But if you do "stop", you must talk to Siri again to set a new timer. For me, this is faster than killing the app with the side buttons.
Now to the passcode: in my experience, the only instance in which the passcode may be required multiple times in a day is when the watch is too loose on the wrist. If the band is not loose on the wrist there should not be anything causing multiple passcode requests. My new Series 5 watch doesn't have that problem and neither did my old Series 3.