2018 F-150 Platinum the issue is first gear to about 5 or 6. If I'm at a dead stop once I start most of the times when it shifts to second or third the engine revs up, the RPM gage goes to 4 or 5 RPM's. This usually happens when I'm driving in slow traffic at about 20-35 MPH. I took it to the dealer a couple of times, and they can't find anything wrong. any suggestions

 

This is to be sure you know and, >not saying< there is no issue.
First gear is a very low ratio so the RPM will go up quite some ways before shifting occurs unless you let up on the throttle more quickly to allow shifting.
Looking at the Tach and hearing the motor rev can be fooling sometimes into thinking the trans is slipping.
Good luck.

 

What Bluegrass stated above is exactly what I became accustomed with doing, letting up off excelletation, allowing a shift to occur. 1st gear will rev up high when you don't. Over time, for me anyway, it seems to have learned and improved or it is I who have learned.

 

Here is an example of the real trans gear ratio spreads.
1st is 4.696 to 1. If the rear axle ratio is 3.55:1 times 4.696 = 16.67 to 1 final ratio.
If the trans quick shifts to 3rd, the ratio becomes 2.146 times 3.55 = 7.6183 as the final ratio.
It can be seen the ratio spread becomes very large at 16.67 to 7.6183.
The engine RPM follows accordingly along the sound.
.
Ford has done this with the transmissions all the way back to the mid 80s AOD, 4R70 etc.
Some modifiers have used an optional gear set in these transmissions to get a closer ratio spread for racing applications as some cars were equipped with these closer ratios.
In these trucks, the 1st gear ratio when in >4wd low< becomes a crawling gear. A sales point for off road use competing with other mfgers when E-Lock is turned on, for that application.
Another point is, at rest, the trans lockup clutch is off. As soon as rpm and speed is satisfied in program, the lock up occurs and should stay on for the rest of the gear changes unless the program decides it should be unlocked at special times and conditions.
This offers a more positive shift feel and reduces clutch wear plus helps fuel mileage by eliminating Converter slip.
The lock up Clutch is a large Cone shape instead of the traditional flat Clutch.

 

The axle code given on the mfg. Sticker of my truck is 15 which I think means an axle ratio 3.15. That's high compared to most I believe. My tires are factory original 245/70R17.

 

For clarification the truck starts from a stop in 8th gear then after 7-10 seconds of acceleration, creeping slowly forward, it will slam into 3rd gear and then operate normally. I’ve uploaded a video of it.

 

Obviously it's a bazaar fault.
What have you done about it?
A thread break-in reply like this one, with your problem, is not helpful to anyone.
There is a converter, a lock up clutch, 10 solenoids, 10 gears combinations, speed sensor and a program module to drive them plus 5 shift modes.
What would you like to hear, as the fault?

 

Check the fluid level per service manual procedure. Some are overfilled, some underfilled. Mine was fortunately right on.

Then reset adaptive shift learn procedure with Forscan.

Probably not related to your issue but how many miles? Change the fluid and/or filter.
I do not have any of the issues you describe, shifts like a dream.