Take it to a different dealer or a reputable mechanic. I'm pretty sure that the ECU has some codes recorded that can be read with appropriate code reader (not a cheap one that only reads basic engine codes). This would be the starting point for diagnostics.
Also, your car has 78k miles. That's quite a lot for the battery with a start-stop system. Although your battery isn't that old time-wise, with the number of start-stop cycles it may be on its last legs. I suggest you check it again (many shops that sell batteries will do it for free).

 

The majority of people with your problem found it was because their batteries were low voltage--either old, or a dead cell, or the DCM drained them down (in the 2019s and 2020s) or the alternator did not charge the battery correctly. As you probably already know, when the battery is discharged the ECM shuts down the auto stop/start function and several other functions to preserve whatever remaining charge the battery has. I second the prior post to get that battery re-tested free at a parts store. I agree that it is probably going bad at 78K miles and 3 years. HOWEVER...When RAV4 batteries are dying you will sometimes (not always) get a MID warning about "Low Volts" and possibly a specific battery or charging warning light. You would also eventually get a whole raft of additional warning lights and MID warnings (EPB malfunction, steering power Low).You are NOT getting these warnings---- There are actually a couple of non-battery mechanical things that might cause your problem: 1. On the 2019-2021 models If the electric wire to the hood latch is broken or disconnected or shorting out that might do it (this is one way people delete that annoying auto S/S.) 2. If the Auto S/S button below the climate control has broken or shorted out that also could do it.

 

Thanks for the feedback guys. I’m at the dealer where I originally purchased the car today hopefully they have a different conclusion. I’ll provide an update.

 

This dealer is saying the battery IS bad and causing the issue. Gonna have them replace it and I’ll see if it comes back.

 

Let us know how much they charge for the battery and installation and the battery model number please.

 

So....the battery tested bad but the technician also found that one of the wires connected to the alternator was frayed pretty badly and causing a voltage issue. They replaced the battery and repaired the frayed wire....Stop/Start works completely normal again.

@doznI4SE everything came out to $210 with labor. I'll get the battery type when I get back to the car.

Next thing is trying to get my diagnostic fee back from the previous dealer who quoted $1400 for a Stop/Start ECU replacement...

 

I have a 2021 rav4, electronic brake light came on while was drive. After car turned off unable to start again. So I tow my car to toyota dealer.
toyota dealer installed wrong battery ( regular battery and not stop and start battery. ) then computer give them a skid ECU no communicate code. Then they replaced ABS ECU and cleared code.. which cost me about 2500$.
Rav4 able to run for 3 days and car stalled when stop at the RED light. Electric parking brake came on again. Unable to start up again. Tow to different toyota dealer. They found out wrong battery was installed. Car start right up with correct battery.