Normal it probably isn't, but it happens occasionally.
Just activate the wipers manually, then return the switch back to intermittent position.
Not a big deal.

 

RX 350 Owner's Manual page 256
Operating the lights and wipers
■ Front door opening linked windshield wiper stop function (vehicles with rain-sensing windshield wipers) When “AUTO” is selected and the windshield wipers are operating,
if a front door is opened, the operation of the windshield wipers will be stopped to prevent anyone near the vehicle from being sprayed by water from the wipers, provided
the vehicle is stopped with the parking brake applied or the shift lever in P. When the front door is closed, wiper operation will resume.

Review your pit stop technique, might you be starting the RX with the door still open? This might trigger the auto stop feature described above.

 

FWIW, I never start the engine before the door is closed, and I get the same behavior as the OP; if the windshield has rain on it the rain-sensing feature does not activate until I do a manual wipe. As others have said, it's not a big deal. Once running the rain-sensing function works and well and I like the ability to adjust its sensitivity.

 

I agree on all points. Not a big deal and I always close the door before starting the engine and yet mine behaves the same way. To the best of my recollection, this has been the behavior with all of my Lexus vehicles with rain-sensing wipers. I just never gave it a thought honestly as its pretty simple to restart. I like the rain sensing wipers and find them more useful than standard intermittent wipers, so I'm fine dealing with this minor quirk.

 

i have all the windows coated don't have to worry when is raining hard anymore

2017 lexus RX 350

 

your windows are coated with what? i use PIAA wiper blades. They are the best in the world. $60 and will last for 6 or 7 years or more.

 

Gtechniq G5, don't have to use my wipers when is raining and 40 mph